Use doc-builder for building and publishing docs

[samsrabin]

Remaining work:

• (Waiting on Add necessary build-docs helpers to release-cesm2.0 branch #142) In version_list.py, replace release-cesm2.0-build-docs with release-cesm2.0.
• (Waiting on Add necessary build-docs helpers to cism-release-cesm2.1 branch #143) In version_list.py, replace cism-release-cesm2.1-build-docs with cism-release-cesm2.1.
• Add GitHub workflows for testing and publishing docs.
• Check that rendered docs all look right! There were multiple extensions in the original conf.py—did I get all of them? doc-builder is only missing sphinx.ext.doctest, which was seemingly never used in the CISM-wrapper docs. Scanning through the rendered docs, I didn't notice anything that looked wrong (except some things that are present in the original docs as well).
• Add PR preview script. Done; see doc/doc-builder/tools/preview_docs_pr
• Enable GitHub Pages for this repo: Settings → Pages → Source: GitHub Actions