Documentation updates

[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.

Change log

• TODO: write change log

Changed

* Miscellaneous documentation improvements
  (`#1786 <https://github.com/glotzerlab/hoomd-blue/pull/1786>`__).

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.

[tommy-waltmann]

I think people are really going to like the copy button!