feat(docs): add more information on PHP reference libraries index

[bshaffer]

We've added https://github.com/googleapis/google-cloud-php/pull/6470, which provides a tool to print (on the CLI) a nice table with all sorts of useful information about each library.

Potentially, this information could be published at https://cloud.google.com/php/docs/reference as well, and contain library version, supported API versions, and release level information.

cc @dandhlee, @vishwarajanand

[dandhlee]

We should sync offline - all of the index pages use the same template, but if there's language specific add-ons we should add for the index pages, I'll be open to adding them in :)

[saranshdhingra]

@dandhlee @bshaffer Just wanted to ask if there's been any update on this or if we're planning to act on this.

[dandhlee]

No information, but I'll follow up with Brent soon.