[Content] Add a simple Microsoft Learn source registry

[troyscott]

Outcome

Record enough provenance to verify and refresh chapters without building a lineage platform.

Acceptance criteria

• Registry stores canonical URL, title, retrieval time, hashes, and chapter mappings.
• Official DP-700 blueprint metadata is represented.
• Registry is human-readable and validated in CI.
• No graph database, lineage service, or generalized dependency engine is introduced.