Automatically update gh-pages documentation on commits and releases

[mdickinson]

Recent changes in Envisage added GitHub Actions workflows for automatically updating the gh-pages branch:

• on every commit to the main branch (so every PR merge for PRs against main), and
• on every published release

I believe the same workflows would be useful on this repository. It would be nice to get this in place for the next major release of Traits.

Envisage PRs : enthought/envisage#565, enthought/envisage#568