feat(defaults): pretty :help headings #30544

Problem:
Headings in :help do not stand out visually.

Solution:
Define a non-standard `@markup.heading.1.delimiter` group and
special-case it in `highlight_group.c`.

FUTURE:
This is a cheap workaround until we have #25718 which will enable:
- fully driven by `vimdoc/highlights.scm` instead of using highlight
  tricks (`guibg=bg guifg=bg guisp=fg`)
- better support of "cterm" ('notermguicolors')

runtime/doc/news.txt

@@ -119,6 +119,9 @@ API
 
 DEFAULTS
 
+• Highlighting:
+  • Improved styling of :checkhealth and :help buffers.
+
 • Mappings:
   • |grn| in Normal mode maps to |vim.lsp.buf.rename()|
   • |grr| in Normal mode maps to |vim.lsp.buf.references()|