XD-2340 Build: Ensure that branch-specific documentation is pulled and generated

* Update `build.gradle`
* Add ability to specify a custom wiki branch

````
    $ ./gradlew pullDocs -PwikiBranch=origin/mycustombranch
````

* By default the doc branch is the same as the XD branch

Jira: https://jira.spring.io/browse/XD-2340
ditto