This allows us to write the changelog reasonably simply.
The biggest downside is that pandoc won't be able to handle it anymore
when converting to markdown, but
sphinx-markdown-builder (https://github.com/codejamninja/sphinx-markdown-builder)
should be able to handle it.
Released under the Python Software Foundation License, this one
doesn't look awful (no green top bar, huzzah!).
Lightly forked it to remove the donation footer (we don't take any)
and to change the python references to fish references.
The image is just our favicon, which is a stylized "f" and therefore
not fantastic (are we facebook?), but it's the best I found, and the
thing before had no images at all.
Fixes#6500
(as far as I'm concerned)
