docs: switch out docgen tool

[jamestrew]

Initial conversion of gendoc to https://github.com/jamestrew/docgen.nvim

main differences from tree-sitter-lua's doc gen:

• uses luacats annotations instead of emmylua allowing for tighter integration between lua_ls type checking and documentation annotations
• all descriptions are parsed as markdown using tree-sitter-markdown so improved parsing
• vimdoc rendering is heavily based off of the code used by neovim core so things will look very similar to neovim core docs