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

docs/default.nix

@@ -1,6 +1,7 @@
 {
   helpers,
   system,
+  nixvim,
   nixpkgs,
   nuschtosSearch,
 }:
@@ -108,6 +109,10 @@ lib.fix (
     gfm-alerts-to-admonitions = pkgs.python3.pkgs.callPackage ./gfm-alerts-to-admonitions { };
 
     man-docs = pkgs.callPackage ./man { inherit options-json; };
+
+    lib-docs = pkgs.callPackage ./lib {
+      inherit nixvim lib;
+    };
   }
   // lib.optionalAttrs (!pkgs.stdenv.isDarwin) {
     # NuschtOS/search does not seem to work on darwin