Add release trigger for gh-pages rebuild

I've noticed that newly released charts don't appear immediately until I manually run the `gh-pages` workflow - Lets see if we can dispatch that on a release event ### Pull Request Checklist <!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request --> * [x] I have added Go unit tests or [Complement integration tests](https://github.com/matrix-org/complement) for this PR _or_ I have justified why this PR doesn't need tests * [x] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately Signed-off-by: `Rhea Danzey <rdanzey@element.io>`