Documentation: Reflect API changes in Spring HATEOAS 1.0+

[rgielen]

The current Spring Data REST reference documentation is not up to date with the API changes introduced in Spring HATEOAS 1.0+.

This pull request updates the sections

• Overriding Spring Data REST Response Handlers
• Customizing the JSON Output

It reflects the following changes in Spring HATEOAS:

• Change Resource to EntityModel
• Change Resources to CollectionModel
• Change ResourceProcessor to RepresentationModelProcessor

It also adds some minor clarifications and fixes to the original docs.

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[gregturn]

Thanks @rgielen!

[gregturn]

Backported to 3.4.x, 3.3.x, and 3.2.x.