[Spike][Docs] reorganize connector reference into reference, how-to guides, and tutorials

[agnes512]

Closes #5961

Suggestion

Currently, our connector reference sits under "Reference Guides". It's hard to find a specific connector page as the path might be Reference Guides/Connector reference/Database Connector/MySQL connector/Tutorial.
Here is a proposal of how to reorganize them into the four quadrants

REFERENCE GUIDES

keep the current connector reference as is.

BEHIND THE SCENES

add a section or a subpage under "Behind the scene/Datasource" that talk about:

• 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

HOW-TOS

add a page "Configuring DataSource"(or a better name) under "How-tos/Access Database" that talks about common connector configurations such as:

• how to configure the connector ( what properties the connector configuration might take
• how type maps ⚠️ should finish task Add missing LoopBack 4 Types docs Add missing LoopBack 4 Types docs #5063 first
  • to database (Database Migration), e.g String -> Varchar, Boolean -> Tinyint
  • from database (Discovery), e.g Bigint -> int, Text -> String
• how to customize table/column names
• how to discover/migration tables/models
• list connector specific topics, e.g transaction, objectId, and readers should check the corresponding connector pages.
• add links to connector reference

TUTORIALS

Add a page "Connect to back-end tutorial" under "Tutorials" that:

• Briefly introduce what is connectors and what connectors LB provides.
• List the connector tutorials we have in connector ref (MySQL, PostgreSQL, Oracle, and MongoDB)
• Add the link to connector ref

[jannyHou]

• "a section or a subpage under Behind the scene/Datasource " 👍
• "add a page "Configuring DataSource"(or a better name) under How-tos/Access Database" 👍
• "add a page Connect to back-end tutorial under Tutorials" 👍

The plan LGTM, awesome.

[bajtos]

I like the proposal at high level 👏🏻

Let's improve the content users will see when this PR is deployed to loopback.io, see my comments below.

[bajtos]

I think the content above (e.g. "Creating a datasource at runtime") should be eventually moved to "How-to guides" and/or the new page "Configuring-datasource.md" you are creating? I guess that's out of scope of this PR, just want to mention it.