Add merge request concepts dev docs

What does this MR do and why?

Describe in detail what your merge request does and why.

In create-stage#12830 (closed) we discussed the confusing terminology of the various pieces of the merge request. This stubs out and gives a place to have a conversation about these items and put some words behind them to solidify terminology across the organization and group.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.