docs: add "help" section with product neutral guides

[bshaffer]

Manually sort the Table of Contents in the reference docs so that the Core libraries such as GAX and Auth and Common Protos are at the top

NOTE: The docs generation pipeline does not support manually sorting certain packages to the top. This may be possible with manually prefixing the ID to come first (e.g. aaa_google_gax)

https://cloud.google.com/php/docs/reference/

This is similar to how .NET does it.

Also consider adding a "Client library help" section. This would collect the following docs:

• AUTHENTICATION
• DEBUGGING
• MIGRATING
• gRPC

And in the future this would contain others such as TROUBLESHOOTING

[bshaffer]

"help" was added in #8077, which can be seen here

We should also add gRPC and Getting Started