Various improvements to the CI setup

[giordano]

• Run jobs only when relevant files are touched
• Set lower timeout for all jobs, to avoid rogue jobs to run needlessly for hours
• Set concurrency groups, to cancel jobs if there are new commits on the same branch
• More informative message for commits pushing docs preview
• Push documentation only on pushes to main

Tagging https://github.com/UCL-ARC/green-team/issues/76 to show we're applying our good practices in terms of CI 🙂