CI enhancements

• Make website a manually triggered job to avoid running a new pipeline.
• Enable pipelines for stable/* and via web.

After this change, there are now three different sets of jobs:

• master (via new commits or web/schedule):

  • build
  • test-baseline
  • website (manual)

• MR to master (detached pipeline):

  • build
  • check (using test-baseline from master)
  • doc

• stable/* (via new commits or web/schedule) and all other branches if triggered via web or schedule:

  • build
  • test
  • doc