update docs for versioning agent config

Issue - Document versioning worksapces agent config (#481721 - closed)

What does this MR do and why?

After we have finished the versioning agent config feature development, we should document on this feature with internal engineer-facing doc. very minimal for customer-facing doc, as this is technical details that should not matter customers.

MR acceptance checklist

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• doc how versioning agent_config works with paper_trail, explain what happened for update/create (between agent_config table and agent_config_version table)
• doc how workspace linked with a given version
• Document how versioning works. E.g., what each of these versions means: 0, 1, >1
• Add developer-facing (https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/remote_development/README.md)
• Add user-facing docs.

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