Docs: adding a connector introduction section to "Datasource"

[agnes512]

Continuation of #6061
Finish https://loopback.io/doc/en/lb4/DataSources.html#connector

Acceptance Criteria

• add contents:
• what is connectors
• what role it plays in req/res cycle, and what relation it has between model and other artifacts (briefly)
• connector types (add links ):
  • SQL (transaction), NoSQL (freeform properties)
  • Services connectors (service proxy) and others
  • (optional) In-memory connector
  • (optional) Community connector

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[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.