CI: run gen_help_html.lua in pull requests

[justinmk]

gen_help_html.lua in the Nvim source is used to generate HTML from vimdoc :help files:

https://github.com/neovim/neovim/blob/6afe5cce7964258f7ee0b25ae08faa551bc76090/scripts/gen_help_html.lua#L766

Currently the full gen() step in gen_help_html.lua is not fully run in Nvim CI because it takes 30-60s. However, 30-60s in the CI for tree-sitter-vimdoc could be acceptable. And it would add a lot of coverage from an actual consumer.

However, it would cause CI here to fail if we rename nodes. That would require a corresponding PR to update gen_help_html.lua.