Fix doc build for Sphinx 4.0

Fix doc build for Sphinx 4.0

Use app.add_css_file() to register krb5.css if possible (it was added
in Sphinx 1.8), since the old name app.add_stylesheet() was removed in
Sphinx 4.0.

Use the highlight directive instead of the highlightlang directive,
which was removed in Sphinx 4.0.

Remove two duplicate table of contents entries to fix warnings.

In the Github Actions configuration, add a second doc build using the
newest version of Sphinx.

ticket: 9006
tags: pullup
target_version: 1.19-next