Show the version of Gradle that apply to a guide

In the longer term this might require more detailed infrastructure work, but for the short term we should add a `docinfo-footer` with text that states something like _Tested with Gradle 3.3_.

- [ ] Update `gs-template` to:
- [ ] Add docinfo.footer with substituble text
- [ ] Update build.gradle file with attributes to allow `docinfosubs : 'gradle-version,gradle-versions-tested'` (See http://asciidoctor.org/docs/user-manual/#attribute-substitution-in-docinfo-files)
- [ ] Revisit existing guides and update accordingly

Start in the gs-template and inspect its build.gradle configuration, then review the Asciidoctor docinfo substitution reference linked in the issue. Add the requested docinfo.footer substitution and build attributes, then revisit existing guides so the footer reflects the Gradle versions tested.

Written by the indexing model from the issue text.