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.