Update documentation.

[joaander]

Description

Make general improvements to the documentation infrastructure:

• Use the latest version of furo
• Update the furo page template if needed.
• Allow keyboard navigation
• Replace deprecated use of readthedocs google analytics
• Add a copy button to code blocks
• Remove references to singularity

Motivation and Context

Ensure that the documentation is up to date as external actors deprecate features.

How Has This Been Tested?

Will check the docs built on readthedocs for this pull request.

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the GSD Contributor Agreement.
• My name is on the list of contributors (doc/credits.rst) in the pull request source branch.
• I have added a change log entry to CHANGELOG.rst.