docs(#8): improve README and documentation

[KyleKing]

Resolves: #8

See discussion: https://github.com/KyleKing/mdformat-admon/pull/4#discussion_r1049566735

• [x] Document what admonition types are supported in README and link to the test fixtures
• [x] Add unit tests for unsupported (Github, etc.)
• [x] Add unit tests for planned-to-be supported (mkdocs?)
• [x] Consider updating the mdit-py-plugin to support full range of mkdocs
• [x] Possibly some additional research to see what else is out there