Requested documentation improvement

Gambit already has maintenance branches named in a predictable way, but we should document this.

Additionally, we want to document the process for applying changes, such as bug fixes, that are common to each of the maintained versions. So far we’ve applied a change to the oldest branch to which it applies (or which is maintained) and merged forward.

• 1) Add a page in developer docs with a table of actively maintained versions and info on branching structure
• 2) Gambit dev team to discuss optimal process for applying changes across versions, then document it on this page
  • Is the current method best or should we use cherry picked commits?
• 3) Either do the following or open a separate issue for later: document a full step by step release workflow for the project, then see if any of this can be automated
• 4) Also include "how to make a release", inclusive of manual PyPI steps for now, see discussion in Add a PyPI GitHub action triggered on release #557