Add prerelease version to non-released branches for ref-guide

The ref-guide build will include all release-branches in the ref-guide build (branch_x_y). This means whenever we cut the branch for the next minor release, it will automatically be added to the official ref-guide. In order to mitigate this, we should include a prerelease tag until the release is complete. This will tell users that it is not a fully released version.

Instead of -SNAPSHOT or -prerelease as the version suffix for all unreleased versions, I think we should go with something shorter that looks better in the drop-down.

for x.0 releases So the first release of a major version, the prerelease suffix should be -alpha and for x.1-99 releases, the suffix should be -beta, since we know it is a bit more stable than the x.0 release.