Refactoring the contents of "EXTENDING LOOPBACK 4" into right places

[jannyHou]

A follow-up story of #5785

Suggestion

A summary of content refactoring:

• Most of existing contents under extending LoopBack 4 goes to "EXTENDING LOOPBACK 4/HOW-TO GUIDES"
• "Build large scale Node.js projects with LoopBack 4" should be a series of shared tutorials between app developer and extension developer. Add them to "EXTENDING LOOPBACK 4/TUTORIALS"
• Extract "Extension point and extensions" into "CONCEPT"
• Add a tutorial page for "Greeter-extension example"

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The new content should look like:

EXTENDING LOOPBACK 4/HOW-TO GUIDES

• Creating components
• Creating decorators
• Contributing REST API
• Creating Services
• Creating Servers
• Extension life cycle
• Extending OpenAPI specification
• Extending Model API builder
• Testing your extension

EXTENDING LOOPBACK 4/TUTORIALS

• Build large scale Node.js projects with LoopBack 4
• Greeter-extension example

CONCEPTS

• Extension point and extensions

Acceptance criteria

• Most of existing contents under extending LoopBack 4 goes to "EXTENDING LOOPBACK 4/HOW-TO GUIDES"
• "Build large scale Node.js projects with LoopBack 4" should be a series of shared tutorials between app developer and extension developer. Add them to "EXTENDING LOOPBACK 4/TUTORIALS"
• Extract "Extension point and extensions" into "CONCEPT"
• Add a tutorial page for "Greeter-extension example" I realized Building large scale projects is using the greeter extension as example. So no need to have a separate page.

[jannyHou]

Closing as PR merged.