Update Distribution MR page with missing info

We have discovered several improvements we can make to the Distribution Team Merge Request Handling handbook page to clarify expectations of reviewers. This came as a result of a meeting with @ibaum on how to ensure that everyone on the team needs to feel comfortable reviewing merge requests. Meeting notes.

Things to do:

• Clarify that it is the responsibility of the maintainer to find another maintainer to look at an MR that is assigned to them if they are unable to. This needs to be spelled out in the doc.
• Specify clearly what is expected of the reviewer in the handbook doc.
• Workflow section needs to be easier to read. Perhaps move workflow to the top of the doc?
• LInk to doc on how to update your team entry to be a reviewer