Reorganize branches and development workflow

[bagofarms]

We're changing up the structure of the project to bring it in line with more popular repositories like Canvas, PHP, and Django. As a result, documentation and CI/CD scripts will need to change. Here is the new structure:
main will be the default branch and represent the latest version, somewhat like a "nightly".
Each release will be in the form stable/major.minor.x.
Issues are handled through branches of the format issue/###-short-description, and are merged into main.
Hotfixes for previous releases will be of the format hotfix/short-description, and will be merged into the appropriate stable branch. That stable branch would then get merged into main and all subsequent stable branches that are affected by the vulnerability.
New docker images will be built for main and all stable branches whenever there are updates.

Thanks to @Thetwam and @jonespm for working with me to come up with this structure.