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
Maybe - I can tune it down - @markpollack Any preferences?. E.g. use gradles logger. Just wanted to be very explicit regarding what is pulled and pushed from where. I mean these could be potentially risky operations.