Remove MkDocs configuration in Sphinx/MyST build

[stevepiercy]

We need to remove the MkDocs configuration files and its droppings from the Sphinx/MyST build. What are all those files? Please add to the following list.

• docs/mkdocs.yml
• docs/theme/partials/header.html
• docs/logos/*
• requirements-docs.txt
• Rename requirements-myst.txt to requirements-docs.txt
• Remove build targets from Makefile
• Update .github/workflows/docs.yml

[stevepiercy]

Fixed in #3070. Closing.