Create table of Message-specific methods for the Message description

Currently the c.g.c pages for Messages combine generated base class methods with Message-specific methods.

For instance for `PhraseSet` part of google.cloud.speech, the base class methods `equals` and `hashcode()` are at the same level as `containsAnnotations` and `getKmsKeyName`.

Ideally, as part of the Enum description at the top of the page, there would be a condensed table of Message-specific methods and the description of what the method does, similar to the tables on the Package overviews.