* RELEASE: update to v0.12

* add myst_deprecations notification to CHANGELOG

* update CHANGELOG

* fix myst-nb intersphinx link

* FIX: broken links to myst-nb due to https://github.com/executablebooks/MyST-NB/pull/365



* add thanks to contributors

* update thanks

* Sphinx 4 in github actions

* upgrade win, os x tests to python=3.8; and pypi publish workflow

* update cache key to be more generic

* use a simple test matrix to set python-version

* FIX: add $

Co-authored-by: Chris Holdgraf <choldgraf@gmail.com>

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

jupytext v0.16 now properly integrates with myst-parser>=0.12

Add note on using other Jupyter kernels

Starts to add all the new functionality from myst-parser and myst-nb.