Migrate To docgen.nvim

https://github.com/jamestrew/docgen.nvim is a documentation generator that is quite similar to Nvim's, using the same luacats internals. It's written by the telescope author and is under active development / use. It generates documentation that is nearly the same, with a few QOL fixes such as automatic links to classes/functions. This is desirable as Nvim's is not public API and has changed a few times e.g. https://github.com/nvim-tree/nvim-tree.lua/pull/3314