Create a publish branch separate from main for gh pages

[ahouseholder]

Based on discussion in PR #301:

Option 2

Alternatively, we could

• clone main to a production branch, and reset the Github Pages publishing
  process to watch production
• merge this PR into main and continue on our merry way

Originally posted by @ahouseholder in #301 (comment)

This issue is to implement the above.

[ahouseholder]

I created a publish branch: https://github.com/CERTCC/SSVC/tree/publish

I chose publish instead of production because it's more specific to the intended use of the branch. We'll use this branch to serve as the source for content that publishes at <https//certcc.github.io/SSVC>. That preserves the option to have a branch named production do something else later if we want it.

publish is currently up-to-date with main.

I added branch protection rules to publish as well, requiring a pull request with at least one review approval.

I also modified the the Github Pages configuration for this repo to publish from the publish branch instead of main.

This is reversible in a matter of minutes, so if we decide we don't like this arrangement later we can put it back the way it was with a few clicks to https://github.com/CERTCC/SSVC/settings/pages