Support publishing of javadocs to gh-pages (in googleapis) as part of grpc package generation

[garrettjonesgoogle]

[garrettjonesgoogle]

This depends on #55 .

[garrettjonesgoogle]

Added a target named updateDocsWithCurrentVersion to the generated build.gradle of each grpc package in googleapis/packman#158 ; as of now, this still needs to be invoked manually.

[shinfan]

This is already done: https://github.com/googleapis/toolkit/blob/master/src/main/resources/com/google/api/codegen/metadatagen/java/grpc/package.snip#L152