doc: split modules into separate pages

[danth]

This splits the generated option documentation into an separate page for each module, which makes it easier for users to browse and discover what is supported. Home Manager and NixOS options for each module are shown alongside each other, which makes it easier to notice when a module is available in both.

The current reference pages for Home Manager and NixOS now only show global options such as stylix.base16Scheme. The URL for these pages has also changed.

This pull request also makes it possible to add hand written documentation about a module, which can be done by creating a file modules/«module»/README.md. The file should follow the format:

# Module Name

Consider describing the application the module targets (if this is not obvious from the name),
how the application needs to be installed for the module to work correctly, which theming items
it supports (colors, fonts, wallpaper, ...), and any caveats the user should be aware of.

<!-- Automatically generated option documentation will be inserted below. -->

[danth]

Follow-up tasks once this is merged:

• Write content for the toplevel Platforms and Modules pages, potentially taken from the current Configuration page
• Support rendering Nix Darwin options
• Support rendering the list of testbeds for each module
• Improve performance by reducing the number of times we call pkgs.nixosOptionsDoc

[trueNAHO]

Can we collapse long categories to avoid scalability issues, similar to NixVim's documentation?

Follow-up tasks once this is merged:

• Support rendering Nix Darwin options

Also the future nixOnDroidModules.stylix options.

• Support rendering the list of testbeds for each module

That would be neat.

• Improve performance by reducing the number of times we call pkgs.nixosOptionsDoc

Yes, generating the documentation takes over one minute to build now.

[danth]

Can we collapse long categories to avoid scalability issues, similar to NixVim's documentation?

Do you mean collapsing the platforms and modules sections? Or separating NixOS/Home Manager options for each target into sub-pages?

[trueNAHO]

Can we collapse long categories to avoid scalability issues, similar to NixVim's documentation?

Do you mean collapsing the platforms and modules sections? Or separating NixOS/Home Manager options for each target into sub-pages?

The current section structure looks good. However, potential future sections following the "Modules" section might be hard to spot with such a large "Modules" index. To resolve this, we might want to treat the "Modules" section differently and collapse it in one of the following ways:

• - Modules
    - *modules* (collapse)
  

• - *Modules* (collapse)
  

If possible, the second approach seems simpler and better.

[danth]

I squashed most of the fixups into the original commits, so this should be suitable to merge with a merge commit to preserve the history.

[danth]

I think everything has been addressed, excluding the follow up tasks.

[trueNAHO]

LGTM, except for some nitpicks.

Feel free to merge after resolving these nitpicks.

Requested changes have been addressed and another review is not required.