Advertise nvim-lua-guide in docs

[clason]

Mention https://github.com/nanotee/nvim-lua-guide at the beginning of :h lua as well as :h lua-vimscript.

Closes #12369 .

[teto]

I am not comfortable with referencing an external website. Some users will keep Neovim 0.5 for years (we have people in the chat using 0.2 ...) so I would prefer an url we control. The github wiki would be nice but even better would be a link to the website IMO.
I would like to add LSP/Treesitter/Lua sections to https://neovim.io/doc/ even if it just redirects to external resources, at least we can provide some context for the URI ("this guide is good for 0.5, this one for 0.6 and so on").

[clason]

Can you pull in https://github.com/nanotee/nvim-lua-guide/blob/master/doc/nvim-lua-guide.txt to https://neovim.io/doc/ ?

[teto]

Looking at it right now

[mhinz]

We can pull it in to a server/site we control and everything, but actually I want to do less. Even if automated, it all adds to the overall complexity.

So, when the repo is active and the community considers it a good resource, let them manage it. And maybe we have more frequent releases in the future, so advertising shouldn't be a big deal anyway. :)

[clason]

Alternatively, we can put a link to it in the README, adding "Lua integration" as a new feature point, then "see :h lua and "?

[teto]

see neovim/neovim.github.io#214 for a follow up