Update community response forum workflow to use new internal category; document forum registration, trust levels

Why is this change being made?

The community response workflow with drafting a forum topic is updated to provide better internal transparency for team members: https://gitlab.com/gitlab-com/marketing/community-relations/community-operations/community-operations/-/issues/139+

This MR documents the new Internal category in the Developer Evangelism response process handbook, and creates a new forum workflow to explain how to edit a topic, in the community operations handbook.

In addition, this MR documents missing forum topics such as

• Registration and Login for wider community members and team members, also explaining oauth
• User Trust levels in general (still linking to Discourse, but adding own words to encourage users)
• Team member trust levels from the oauth group, and manual updates to trust level 4
• Nomination process for community members to trust level 4
• Ensure to balance the SSoT between the two pages; describing categories and trust levels in the forum handbook.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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