Problem:
The html and css of the website's documentation pages are defined in
long strings in gen_help_html.lua, making it hard to maintain and
improve them. E.g. adding in headers that state the documentation is for
nightly Nvim has been a long standing feature request.
Solution:
Move the inlined css and html (e.g. the <head>, <nav>, etc.) to Hugo.
Now that the website is build with Hugo, we can use its templating
system to generate the full html/css from the Tree-sitter tree:
https://github.com/neovim/neovim.github.io/pull/437
b57ed5e940
