Organize packages better in `google-cloud-java` reference docs

[jabubake]

https://googlecloudplatform.github.io/google-cloud-java/0.25.0/apidocs/index.html

: it is confusing to the .spi, .stub , hand-written layer / auto-gen layer all listed together eg for firestore. The top category should only list the packages that the user is expected to use.

[pongad]

@garrettjonesgoogle You told me on IM that .spi should be removed from firestore right? Should .spi.v1beta1 be merged into .v1beta1?

I don't actually think we ever decided on what to do with .stub packages. Should they have their own section in the overview?

[garrettjonesgoogle]

.spi is removed from auto-generated clients, but it's still used for the internals of manual clients. Thus, .spi.v1beta should stay where it is.

We should probably make a new section for .stub packages.