Update "Building the Docs" info

[MoltonMontro]

This PR does three things:

• Updates the README steps for building the docs locally. We can close out #327 if this is merged.

• Enables offline, zipped HTML downloads (alternative to PDF and EPUB3).

• Reenables sphinx-tabs extension (dropped due to deprecation of Sphinx content injection). This hasn't broken anything now, but it's a popular extension that we may want to use when showing multiple code examples. E.g., .dat vs .asset vs json format, inline vs object, multiple examples (of the same language), etc.

[SDGNelson]

Nice improvements, thanks! 3.x works well for the version display. :)

[MoltonMontro]

Hopefully looks better than just seeing a "3" on PR builds 😅