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
fix typo in async docs
Merge pull request #129 from yamkazu/async-typo

fix typo in async docs
GRAILS-9934 added docs for the two ways to customize plugin ZIP/JAR contents
Fix custom foreign key column names

Fixed the example of renaming the column names of a composite foreign
key. The person block outside columns is a invalid mapping.
GRAILS-9758 - document create-app --skip-wrapper
Add some notes on BindUsing
docs on configuring snapshot checking with Aether
Merge pull request #130 from sergiomichels/master

Fix custom foreign key column names
Fix link to Grails API doc
Merge pull request #133 from ganta/fix/grails-api-link

Fix link to Grails API doc
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'
Fixed outdated example, Unit testing controllers, testing duplicate form submission