Refactor `serve-docs` using `makeWrapper`, make it available as a
`serve-docs` package and as the `docs` "app".
This means `nix build .#docs` will build the docs while `nix run .#docs`
will run the server.
Allows using the `tests` devshell command to discover the full attrpath
of a test.
e.g.
```
$ tests --attr modules-lsp plugins-by-name-lazygit
Printing 2 tests: modules-lsp plugins-by-name-lazygit
Full attr paths:
- checks.x86_64-linux.test-2.entries.modules-lsp
- checks.x86_64-linux.test-18.entries.plugins-by-name-lazygit
```
This is much faster than running `nix fmt`.
Running treefmt directly will use the treefmt installed in the current
shell. Running `nix fmt` will first re-evaluate the flake.
This removes the need for end-users to manually set
`nixvim.inputs.devshell.follows = ""` (etc)
We offload evaluation of some of our flake modules into a `dev`
partition submodule.
- When its not needed, this submodule is not evaluated.
- When it is needed, it fetches extra inputs from `flake/dev/flake.nix`
as part of evaluating the submodule.
See https://flake.parts/options/flake-parts-partitions.html
