Add CI jobs to build docs (#6413)

Add CI jobs to build docs (#6413)

This PR adds CI jobs to automatically build documentation for branches on the main repository and for pull request.
The first job builds the documentation for every push and pr and stores the generated documentation in cvc5.github.io/docs-ci. All versions are stored and for every branch and PR a symbolic link to the most recent version is maintained.
Note that the PR jobs are run by the pull_request_target trigger that allows access to repository secrets, but runs in the context of the target branch and we thus need to (carefully!) pull in the relevant changes manually.
The second job runs once a week and prunes the docs-ci repository: all directories that have not been touched for a month are removed, and all commits older than a month are squashed into a single commit. This retains the full history for the last month, but effectively removes everything older than that.