upgrade plugin documentation

[ctb]

Dev docs: https://sourmash.readthedocs.io/en/latest/dev_plugins.html -

• suggest adding plugins to https://github.com/ctb/sourmash-mixers
• suggest plugins be added whenever new dependencies would be needed

CLI docs: https://sourmash.readthedocs.io/en/latest/command-line.html#using-sourmash-plugins -

• link to sourmash-mixers together with a few examples of super cool things you can do?
• revise text etc etc

[ctb]

also:

• provide suggestions for documentation & metadata for authors:
• provide guidance on how to test your CLI plugin at the CLI (minimal testing regime: sourmash scripts, look for description etc.)

[ctb]

sourmash-mixers is now live on pypi: https://pypi.org/project/sourmash-mixers/0.2/ 🎉

There's a nice issue template on that repo that helps guide new plug-in development, too.

[ctb]

add a checklist for plugin deprecation, too? per #3285