Use the sphinx-better-subsection extension to use explicit targets for section permalinks

[tsibley]

As that Python package requires ≥3.7, this theme will now too. That's acceptable in the development contexts in which we generate docs, which should be using newer Pythons anyway.

c.f. https://github.com/nextstrain/docs.nextstrain.org/commit/57fe1888#all_commit_comments

Testing

• [x] Demos work
• [x] Checks pass