Push updated documentation through CI jobs

[giacomofiorin]

Support updating the documentation directly from GitHub Actions. Currently, the doc Makefile already uses the current Git branch (or tag) to define the version of the code (added in #606).

This PR adds the necessary plumbing to push the documentation just built to the website's repository. In order to do so, we need the following:

1. The commit message must include the string "[update-doc]"
2. This commit must be at the head of a push, so that the CI jobs are triggered correctly.

The documentation is added to the website in a sub-folder with the same name as the branch, so that we can preview the documentation in a PR for review purposes. In between PRs, we may want to remove commits for these temporary branches from the website repository to avoid growing its size too much.

When pushing to master, Doxygen updates are pushed in addition to the manual.

I have already verified that the website was correctly updated by generating pages like this:
https://colvars.github.io/push-doc/colvars-refman-namd.html
Will now delete those files from the website repository before attempting to merge into master.