DNM/WIP: user-docu: generate docs using treesitter

[justinmk]

Problem:

The HTML :help docs generated by the old doc_html task (which is driven by an old awk script runtime/doc/makehtml.awk). Besides being hard to maintain (ad hoc parser and no one has touched it in decades), it has bugs like:

• https://github.com/neovim/neovim.github.io/issues/96
• https://github.com/neovim/neovim.github.io/issues/97

Solution:

Use the gen_help_html.lua script from https://github.com/neovim/neovim/pull/11967

There are some regressions, so currently this generates to a new temporary user2/ path so that people can try it out.