Add language tabs on code blocks

[Inscrutable]

@gabizou has requested language tabs for different code blocks. In the future, Sponge's workspaces for development will be moving to Kotlin-DSL. SpongeDocs will need to be able to provide code examples how to use it between gradle, maven, and kotlin-dsl. Tabs are a way to manage this neatly.

The following third-party Sphinx add-on does the job: https://github.com/djungelorm/sphinx-tabs

[phit]

I can think of quite a few other places where tabs could be useful to cut down page length e.g. OS specific stuff

[Grauldon]

Sweet! IDEs are another area this can help. How far in the future?