gh-pages branch commit history length impacts performance

[ChrisBAshton]

The gh-pages branch gets built every hour, and each commit is quite sizeable. Over time, this commit history amounts to many MBs (even GBs), making it slow to clone the repository.

Version history for the gh-pages branch isn't particularly important; it's simply a compiled output of lots of sources. So we'd like to minimise the amount of space the gh-pages history takes up.

An attempt was made in #3321 to 'orphan' all but the latest commit on gh-pages, drastically reducing its size. However, the feature requires force-pushing, which is not allowed under the 'branch protection rules' for the gh-pages branch, which we do not want to weaken.

So we reverted the PR in #3323.

This is a known problem, with a proposed solution of using a bot account for GitHub Actions, and locking down force-pushes to only the bot account.