Proposal: Add directory recommendation

What does this MR do and why?

Describe in detail what your merge request does and why.

Add a recommendation for reactive processor location in docs.

Processors specific to a group should live under a group directory in triage/processor/.

General processors should live under triage/processor/.

This could help organize code and find groups specific processors as well could improve the experience for self-serving groups in building their own processors.

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 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