Remove Bulma CSS classes from HTML output (Bulma no longer used)

[tony]

Summary

The HTML writer and TOC helpers emit Bulma-specific CSS classes, but the
project no longer uses Bulma. These are dead styling hooks and should be removed
or replaced with framework-neutral classes.

Where

• DjangoDocutilsHTMLTranslator emits title is-1, subtitle,
  topic-title first, sidebar-title, and a table class; an inline comment
  still reads "for bulma css":
  https://github.com/tony/django-docutils/blob/v0.29.0/src/django_docutils/lib/writers.py
• TOC scaffolding emits fixed-toc-menu menu and menu-label:
  https://github.com/tony/django-docutils/blob/v0.29.0/src/django_docutils/lib/publisher.py
• TOC contents emit menu-list:
  https://github.com/tony/django-docutils/blob/v0.29.0/src/django_docutils/lib/transforms/toc.py
• The file role styling notes reference Bulma:
  https://github.com/tony/django-docutils/blob/v0.29.0/src/django_docutils/lib/roles/file.py

Suggested approach

Decide on replacement classes (or none), update the translator and TOC helpers
accordingly, and refresh any docs CSS that targeted these classes.