Build and deploy to GitHub Pages from GitHub Actions.

Change-Id: I4140492ee8ff367bbe65dd27f7dbd11f6fc7ad6f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62610
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>