GitHub extension deprecation

[facelessuser]

Let's be honest, the github extension is not a true GFM emulator. And even though it includes extensions that come closish to GitHub features, it doesn't, and will never, emulate true GFM. Further, I have no intentions of following changes of GFM and ensuring we are synced up. Recent repository link shorthand feature is GitHub like, but honestly closer resembles GitLab.

The GitHub extension represents something that is really no longer a goal in any way, shape, or form for pymdown-extensions. Yes some features are inspired by GitHub, but I'm not looking to match GitHub.

Let's deprecate the extension and let people pick and choose what extensions they like. And if they pick extensions that closely resemble GitHub's GFM, then that is fine.