Merge branch 'refactor-release-notes' into 'master' 3.1.0b2

Create sub-sections for different release series

When looking at the documentation, the release notes section on the
sidebar was incredibly long, due to the number of releases available.
To make this more usable, I've split the release notes index into
sections for each release series (e.g., 3.x, 2.x, etc.). This allows
documentation users to open the release notes sidebar section, and then
select which series they want. That will expand and let them pick the
version.

See merge request !145

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