docs: Create a few code snippets as examples for using Spanner Graph …

…using Java (#3234)

* Create a set of code snippets for using Graph on Cloud Spanner using Java

* Update SpannerGraphSample to align with doc examples already created for gcloud and cli

* Fix syntax for upate with graph query in dml

* Remove all references to Gender

* Apply the google-java formatter to fix style

* Add integration tests for SpannerGraphSample

* Fix some comments in SpannerGraphSample.java

* Fix naming style issues

* Fix one region tag that was inconsistently named

* Fix style issue in test code