Genericize codegen pages and add JVM

[stuhood]

Currently, our documentation is laid out as $codegen x $lang, which results in a lot of redundant information (i.e. the setup steps from https://www.pantsbuild.org/docs/protobuf-python and https://www.pantsbuild.org/docs/protobuf-go share 3 or 4 steps).

We should add dedicated pages per $codegen impl, which contain generic information about each generator. Per $lang, we should then have a table (or header-ed section) of the differences between $codegen impls for them.

We should then document protobuf support for Java/Scala.

[Eric-Arellano]

Alternatively, we can leverage in-repo docs to DRY the same snippet over multiple pages. That is advantageous because users only need to read precisely what they care about. But, that's hard to do with readme.com and facilitated by something like Sphinx with its include directives