Change release guidance and logic to just use master

[matgrioni]

Having a separate develop branch seemed like a good idea at some point and made sense given my engineering experience up to that point, but I think a simpler and sufficient approach would be to just use master and tagged changes. The latest state of master wouldn't necessarily correspond to any particular released version, but it is easier for contributors, makes the release process easier and less likely to result in errors.

The documentation configuration may also need to be adjusted to match this new format (i.e. stable is actually the latest tag not the head of master).