Add development guidelines for PushRules

What does this MR do and why?

Add development guidelines for PushRules

Adds business logic locations, entrypoints, and flow diagram for Push Rules feature.

Part of https://gitlab.com/gitlab-com/feature-change-locks/-/issues/49+

Screenshots or screen recordings

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Before	After

How to set up and validate locally

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

MR acceptance checklist

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