Update Threat Insights refinement policy

What does this MR do and why?

Our refinement policy can be heavy handed at times, continuously assigning someone on issues that we are intentionally waiting to be picked up. This adds a rule to not auto assign issues with the ~automation:no assign label.

It also fixes the link to our handbook, which changed a few weeks ago.

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-with-a-dry-run on how to perform dry-runs.

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