Move the mdbook menu rendering code into the module system and
generalise it to apply to multiple "categories" (mdbook parts) and
"types" of category (prefix, suffix, etc).
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
