Asset Processor - Add metadata maintainers documentation

[amzn-mike]

Change summary

Added documentation on the metadata relocation system for devs/maintainers

Submission Checklist:

• Descriptive active voice - Do descriptive sentences have a clear subject and action verb?
• Answer the question at hand - Does the documentation answer a what, why, how, or where type of question?
• Consistency - Does the content consistently follow the Style Guide?
• Help the user - Does the documentation show the user something meaningful?

[AMZN-stankowi]

This is great, we don't have a lot of examples yet of engine maintainer documentation, so thanks for putting this together, I think it will help establish patterns we use for future engine maintainer documentation.