Improve reference documentation on Resources

[sbrannen]

Status Quo

As discussed in #26443, the Resources section of the reference documentation is very outdated, focusing primarily on XML-based configuration of the resource locations for the ApplicationContext and not directly mentioning ResourcePatternResolver at all.

Deliverables

• Document the ResourcePatternResolver API as an alternative to the ResourceLoader API.
• Document PathResource as a built-in Resource.
• Consider providing examples where resources are injected into fields of type Resource or Resource[] in @Configuration classes or annotated components via @Autowired or @Value semantics.

[sbrannen]

@nightswimmings, I've completed all planned changes to the Resources section of the reference manual.

Please take a look and let us know if we've addressed all of the concerns you raised in #26443.