docs: Correctly generate documentation for types either a (submodule ...) (#929)

This requires pulling quite a lot of code from nixpkgs in order to
make the `either` type work correctly, the effects can be seen for
example in the documentation of `ollama.promps` and `ollama.actions`

docs/man/default.nix

@@ -0,0 +1,19 @@
+{
+  options-json,
+  runCommand,
+  installShellFiles,
+  nixos-render-docs,
+}:
+runCommand "nixvim-configuration-reference-manpage" {
+  nativeBuildInputs = [installShellFiles nixos-render-docs];
+} ''
+  # Generate man-pages
+  mkdir -p $out/share/man/man5
+  nixos-render-docs -j $NIX_BUILD_CORES options manpage \
+    --revision unstable \
+    --header ${./nixvim-header.5} \
+    --footer ${./nixvim-footer.5} \
+    ${options-json}/share/doc/nixos/options.json \
+    $out/share/man/man5/nixvim.5
+  compressManPages $out
+''

docs/man/nixvim-footer.5

@@ -0,0 +1,3 @@
+.SH "AUTHORS"
+.PP
+nixvim maintainers

docs/man/nixvim-header.5

@@ -0,0 +1,15 @@
+.TH "NIXVIM" "5" "01/01/1980" "Nixvim" "Nixvim Reference Pages"
+.\" disable hyphenation
+.nh
+.\" disable justification (adjust text to left margin only)
+.ad l
+.\" enable line breaks after slashes
+.cflags 4 /
+.SH "NAME"
+nixvim options specification
+.SH "DESCRIPTION"
+.PP
+This page lists all the options that can be used in nixvim. Nixvim can either be used as a Home-Manager module, an NixOS module or a standalone package. Please refer to the installation instructions for more details.
+.SH "OPTIONS"
+.PP
+You can use the following options in a nixvim module.