[Spike] docs: untangle connector reference into reference, how-to guides and tutorials

[dhmlau]

Originated from: #5786

Part of #5960

Acceptance Criteria

• Review the existing connector reference contents and create a plan on how to extract out content into reference, how-to guides and tutorials.
  • As the outcome of this spike, once agreed upon, follow up stories will be created under epic docs: Rework connector reference contents  #5960.

[agnes512]

Discussion from the estimation meeting:

Can follow the process or have a high-level docs to talk about the commonalities of connectors (learning tree of connectors:

• what is connectors
• what role it plays in req/res cycle, and what relation it has between model and other artifacts
• connector types:
  • SQL (transaction), NoSQL (freeform properties)
  • Services connectors (service proxy) and others
• How-to (in common)
  • how type maps
  • how to customize table/column names
  • how to discover/migration tables/models
• Explain challenges

Candidates for references should be specific contents, e.g handling objectId (MongoDB)