Improve docs on Accept header and content negotiation.

The user guide did not make it clear that Grails ignores the Accept header for
content negotiation unless explicitly configured to do so. Hence why the issue
GRAILS-9810 was raised.

I have now explained how to activate this feature and also how best to use it,
i.e. don't use `withFormat()` for HTML pages.
Added warning to install-plugin about using BuildConfig instead
Merge branch '2.1.x' into 2.2.x
GRAILS-10032 - Clarify that a JDK is required
Merge branch '2.1.x' into 2.2.x
Merge branch '2.2.x'
Clarify usage of the 'mappedBy' domain class property.

The existing documentation did not explain why you might want to use the
'mappedBy' property for many-to-one or one-to-one associations (only for
one-to-manies). This has now been added. I have also highlighted that a
developer can use values of "none" to indicate that an association is
unidirectional rather than bidirectional.
Merge branch '2.1.x' into 2.2.x
Merge branch '2.2.x'
some docs about @BindingFormat
add some notes about the data binder in whatsNew23
document custom ValueConverters
fix some api links
Fixed wrong doc, claiming to show show.gsp on action list
Merge branch '2.1.x' into 2.2.x
Merge branch '2.2.x'