Add formatting for blockquotes

[bmaupin]

See https://bmaupin.github.io/wiki/applications/multimedia/gmtp.html

Sphinx RTD theme just does an indentation: https://sphinx-rtd-theme.readthedocs.io/en/stable/demo/demo.html#block-quotes

But it might be nice to have some kind of additional highlighting, e.g.

• https://github.com/bmaupin/wiki/blob/master/applications/multimedia/gmtp.md
• https://docs.gitlab.com/12.10/ee/user/markdown.html#blockquotes
• https://docs.gitlab.com/14.6/ee/user/markdown.html#blockquotes
• https://vsoch.github.io/docsy-jekyll/docs/getting-started#quotes