Moved all function-specific docs from `docs/lib/index.md` into RFC145
doc-comments.
Added `lib.nixvim.lua.toLuaObject` to hold the public docs and serve as
a stable impl of `toLua'` in case we decide to change its defaults.
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
