refactor: move nixos-manual to reference/ + graphifyignore

reference/nixos-manual/contributing-to-this-manual.chapter.md

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+# Contributing to this manual {#chap-contributing}
+
+The sources of the NixOS manual are in the [nixos/doc/manual](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual) subdirectory of the [Nixpkgs](https://github.com/NixOS/nixpkgs) repository.
+This manual uses the [Nixpkgs manual syntax](https://nixos.org/manual/nixpkgs/unstable/#sec-contributing-markup).
+
+You can quickly check your edits with the following:
+
+```ShellSession
+$ cd /path/to/nixpkgs
+$ $EDITOR doc/nixos/manual/... # edit the manual
+$ nix-build nixos/release.nix -A manual.x86_64-linux
+```
+
+If the build succeeds, the manual will be in `./result/share/doc/nixos/index.html`.
+
+There's also [a convenient development daemon](https://nixos.org/manual/nixpkgs/unstable/#sec-contributing-devmode).
+
+The above instructions don't deal with the appendix of available `configuration.nix` options, and the manual pages related to NixOS. These are built, and written in a different location and in a different format, as explained in the next sections.
+
+## Development environment {#sec-contributing-development-env}
+
+In order to reduce repetition, consider using tools from the provided development environment:
+
+Load it from the NixOS documentation directory with
+
+```ShellSession
+$ cd /path/to/nixpkgs/nixos/doc/manual
+$ nix-shell
+```
+
+To load the development utilities automatically when entering that directory, [set up `nix-direnv`](https://nix.dev/guides/recipes/direnv).
+
+Make sure that your local files aren't added to Git history by adding the following lines to `.git/info/exclude` at the root of the Nixpkgs repository:
+
+```
+/**/.envrc
+/**/.direnv
+```
+
+### `devmode` {#sec-contributing-devmode}
+
+Use [`devmode`](https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/de/devmode/README.md) for a live preview when editing the manual.
+
+## Testing redirects {#sec-contributing-redirects}
+
+Once you have a successful build, you can open the relevant HTML (path mentioned above) in a browser along with the anchor, and observe the redirection.
+
+Note that if you already loaded the page and *then* input the anchor, you will need to perform a reload. This is because browsers do not re-run client JS code when only the anchor has changed.
+
+## Contributing to the `configuration.nix` options documentation {#sec-contributing-options}
+
+The documentation for all the different `configuration.nix` options is automatically generated by reading the `description`s of all the NixOS options defined at `nixos/modules/`. If you want to improve such `description`, find it in the `nixos/modules/` directory, and edit it and open a pull request.
+
+To see how your changes render on the web, run again:
+
+```ShellSession
+$ nix-build nixos/release.nix -A manual.x86_64-linux
+```
+
+And you'll see the changes to the appendix in the path `result/share/doc/nixos/options.html`.
+
+You can also build only the `configuration.nix(5)` manual page, via:
+
+```ShellSession
+$ cd /path/to/nixpkgs
+$ nix-build nixos/release.nix -A nixos-configuration-reference-manpage.x86_64-linux
+```
+
+And observe the result via:
+
+```ShellSession
+$ man --local-file result/share/man/man5/configuration.nix.5
+```
+
+If you're on a different architecture that's supported by NixOS (check file `nixos/release.nix` on Nixpkgs' repository) then replace `x86_64-linux` with the architecture. `nix-build` will complain otherwise, but should also tell you which architecture you have + the supported ones.
+
+## Contributing to `nixos-*` tools' manpages {#sec-contributing-nixos-tools}
+
+The manual pages for the tools available in the installation image can be found in Nixpkgs by running (e.g for `nixos-rebuild`):
+
+```ShellSession
+$ git ls | grep nixos-rebuild.8
+```
+
+Man pages are written in [`mdoc(7)` format](https://mandoc.bsd.lv/man/mdoc.7.html) and should be portable between mandoc and groff for rendering (except for minor differences, notably different spacing rules.)
+
+For a preview, run `man --local-file path/to/file.8`.
+
+Being written in `mdoc`, these manpages use semantic markup. This following subsections provides a guideline on where to apply which semantic elements.
+
+### Command lines and arguments {#ssec-contributing-nixos-tools-cli-and-args}
+
+In any manpage, commands, flags and arguments to the *current* executable should be marked according to their semantics. Commands, flags and arguments passed to *other* executables should not be marked like this and should instead be considered as code examples and marked with `Ql`.
+
+- Use `Fl` to mark flag arguments, `Ar` for their arguments.
+- Repeating arguments should be marked by adding an ellipsis (spelled with periods, `...`).
+- Use `Cm` to mark literal string arguments, e.g. the `boot` command argument passed to `nixos-rebuild`.
+- Optional flags or arguments should be marked with `Op`. This includes optional repeating arguments.
+- Required flags or arguments should not be marked.
+- Mutually exclusive groups of arguments should be enclosed in curly brackets, preferably created with `Bro`/`Brc` blocks.
+
+When an argument is used in an example it should be marked up with `Ar` again to differentiate it from a constant. For example, a command with a `--host name` option that calls ssh to retrieve the host's local time would signify this thusly:
+```
+This will run
+.Ic ssh Ar name Ic time
+to retrieve the remote time.
+```
+
+### Paths, NixOS options, environment variables {#ssec-contributing-nixos-tools-options-and-environment}
+
+Constant paths should be marked with `Pa`, NixOS options with `Va`, and environment variables with `Ev`.
+
+Generated paths, e.g. `result/bin/run-hostname-vm` (where `hostname` is a variable or arguments) should be marked as `Ql` inline literals with their variable components marked appropriately.
+
+ - When `hostname` refers to an argument, it becomes `.Ql result/bin/run- Ns Ar hostname Ns -vm`
+ - When `hostname` refers to a variable, it becomes `.Ql result/bin/run- Ns Va hostname Ns -vm`
+
+### Code examples and other commands {#ssec-contributing-nixos-tools-code-examples}
+
+In free text names and complete invocations of other commands (e.g. `ssh` or `tar -xvf src.tar`) should be marked with `Ic`, fragments of command lines should be marked with `Ql`.
+
+Larger code blocks or those that cannot be shown inline should use indented literal display block markup for their contents, i.e.
+
+```
+.Bd -literal -offset indent
+...
+.Ed
+```
+
+Contents of code blocks may be marked up further, e.g. if they refer to arguments that will be substituted into them:
+
+```
+.Bd -literal -offset indent
+{
+  config.networking.hostname = "\c
+.Ar hostname Ns \c
+";
+}
+.Ed
+```