Autogenerate docs/index.rst for un-versioned APIs

[parthea]

File docs/index.rst is currently handwritten for un-versioned APIs such as google-apps-script-type, google-cloud-common and google-shopping-type. We should update the code generator so that we can use the autogenerated version.

For example for google-apps-script-type, see 563a299 with the correct version

[parthea]

Transferring to google-cloud-python as the code for gapic-generator will be migrated there.