Update bot thank you links

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR updates three links in the gitlab-bot thank you message.

• Fixes the Get Help link by pointing to them on docs.gitlab.com
• Removes the missing header for #contribution-flow on the contribution docs link
• Removes the link to merge request workflow (which also had a broken jump link) since this page is already linked from the contribution docs link before it and is currently outdated

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review