Update CONTRIBUTING.adoc for hosted Haddock docs

[catch-21]

Pre-submit checklist:

• Branch
  • [ ] Tests are provided (if possible)
  • [ ] Commit sequence broadly makes sense
  • [ ] Key commits have useful messages
  • [ ] Formatting, PNG optimization, etc. are updated
  • [ ] Important changes are reflected in changelog.d of the affected packages
• PR
  • [ ] Self-reviewed the diff
  • [ ] Useful pull request description
  • [ ] Reference the ADR in the PR and reference the PR in the ADR (if revelant)
  • [ ] Reviewer requested