Update getting started with alternative gradle instructions

[philwebb]

Our getting started section in the docs currently uses Maven. It would be nice to offer a Gradle version alongside it.

[vpavic]

I think that Gradle instructions should guide users to set up the Gradle Wrapper, which is the recommended way of executing Gradle builds. The change that resolved this issue has no mention of Wrapper at all.

[mhalbritter]

I intended to keep the Getting started instructions as simple as possible, and I don't think that the wrapper provides much benefit here. When bootstrapping the project with start.spring.io, the wrapper is included.