docs: lsp, misc

- Problem: It's not clear for new plugin developers that `:help` uses
  a help-tags file for searching the docs, generated by `:helptags`.
  - Solution: Hint to the |:helptags| docs for regenerating the tags
    file for their freshly written documentation.

Co-authored-by: Yochem van Rosmalen <git@yochem.nl>

src/gen/gen_help_html.lua

@@ -73,6 +73,7 @@ local new_layout = {
   ['intro.txt'] = true,
   ['lua.txt'] = true,
   ['lua-guide.txt'] = true,
+  ['lua-plugin.txt'] = true,
   ['luaref.txt'] = true,
   ['news.txt'] = true,
   ['news-0.9.txt'] = true,