gen_help_html: truncate parse-error sample text

Enforce consistent terminology (defined in
`gen_help_html.lua:spell_dict`) for common misspellings.

This does not spellcheck English in general (perhaps a future TODO,
though it may be noisy).

All parser errors have been fixed; make sure we don't introduce new
ones.

Remove the user-manual ToC from help.txt, because:
1. it duplicates usr_toc.txt
2. it is not what most readers are looking for in the main help page.

fix https://github.com/neovim/tree-sitter-vimdoc/issues/49
fix https://github.com/neovim/tree-sitter-vimdoc/issues/50
fix https://github.com/neovim/tree-sitter-vimdoc/issues/51

Note: although the tolerance in help_spec.lua increased, the actual
error count with the new parser decreased by about 20%. The difference
is that the old ignore_parse_error() ignored many more errors with the
old parser.

fix https://github.com/neovim/tree-sitter-vimdoc/issues/37
fix https://github.com/neovim/tree-sitter-vimdoc/issues/44
fix https://github.com/neovim/tree-sitter-vimdoc/issues/47

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

- Docs HTML: improvements in https://github.com/neovim/tree-sitter-vimdoc
  allow us to many hacks in `gen_help_html.lua`.
- Docs HTML: support nested lists.
- Docs HTML: avoid extra newlines (too much whitespace) in old
  (preformatted) layout.
- Docs HTML: disable golden-grid for narrow viewport.
- Workaround for https://github.com/neovim/neovim/issues/20404

closes https://github.com/neovim/neovim/issues/20404

Generate correct leading whitespace for argument, taglink, tag, etc.