docs/lib: init

Generate reference docs for functions that have RFC145 style
doc-comments.

1. function locations

  `docs/lib/function-locations.nix` scans nixvim's extended lib,
  extracting "position entries" via `unsafeGetAttrPos`.

  This is then converted into a nixdoc `locations.json` map of
  "function name" → "markdown location string".

2. mdbook menu

  `docs/lib/menu.nix` renders a mdbook menu representing all page
  entries.

3. markdown pages

  `docs/lib/default.nix` expects a set of "page entries", which come
  from `docs/lib/pages.nix` by default. It passes this data to
  `function-locations.nix` and `menu.nix`, and uses it internally to
  render markdown pages.

  Page entries can contain a `file` to render using `nixdoc`, and also a
  `markdown` attribute which will be included at the top of the docs.

  Additionally, a `title` can be included. This forms the heading
  `$name: $title`, where `name` is derived from the page's attr-path.

  See https://github.com/nix-community/nixdoc

flake/packages.nix

@@ -1,4 +1,5 @@
 {
+  self,
   inputs,
   helpers,
   ...
@@ -12,6 +13,7 @@
     }:
     {
       packages = import ../docs {
+        nixvim = self;
         inherit helpers;
         inherit system;
         inherit (inputs) nixpkgs;