[Backport release-0.8] fix(docs-html): keycodes, taglinks, column_heading (#20590)

fix(docs-html): keycodes, taglinks, column_heading

Problem:
- Docs HTML: "foo ~" headings (column_heading) are not aligned with
  their table columns/contents because the leading whitespace is not
  emitted.
- taglinks starting with hyphen like |-x| were not recognized.
- keycodes like `<foo>` and `CTRL-x` were not recognized.
- ToC is not scrollable.

Solution:
- Add ws() to the column_heading case.
- Update help parser to latest version
  - supports `keycode`
  - fixes for taglink, argument
- Update .toc CSS. https://github.com/neovim/neovim.github.io/issues/297

fix https://github.com/neovim/neovim.github.io/issues/297

(cherry picked from commit 6d74676848)

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

test/functional/lua/help_spec.lua

@@ -21,7 +21,7 @@ describe(':help docs', function()
     ok(rv.helpfiles > 100, '>100 :help files', rv.helpfiles)
     -- Check that parse errors did not increase wildly.
     -- TODO: Fix all parse errors in :help files.
-    ok(rv.err_count < 280, '<280 parse errors', rv.err_count)
+    ok(rv.err_count < 150, '<150 parse errors', rv.err_count)
     eq({}, rv.invalid_links, exec_lua([[return 'found invalid :help tag links:\n'..vim.inspect(...)]], rv.invalid_links))
   end)
 
@@ -43,7 +43,7 @@ describe(':help docs', function()
       tmpdir
     )
     eq(4, #rv.helpfiles)
-    ok(rv.err_count <= 6, '<=6 parse errors', rv.err_count)
+    ok(rv.err_count <= 1, '<=1 parse errors', rv.err_count)
     eq({}, rv.invalid_links, exec_lua([[return 'found invalid :help tag links:\n'..vim.inspect(...)]], rv.invalid_links))
   end)
 end)