Exclude docs-only issues from Threat Insights policies

What does this MR do and why?

Update two Threat Insights policies to take into account the existence of docs-only issues.

• Example for policies/groups/gitlab-org/threat-insights/issue-weights.yml (Put engineering issues missing weight back into refinement)
• Example for policies/groups/gitlab-org/threat-insights/tech-stack-label.yml (Add missing tech stack labels to issues ready for development)

The policies above will no longer apply to issues labelled with docs-only.

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