Fix name_pretty and templates so generated documentation reads smoothly

[vchudnov-g]

In the synthtool Python templates ,the docs/summary_overview.md template uses {{ name_pretty }} API in the text, while the README template only says Python client for {{ name_pretty }}

In Google Cloud Python, the .repo-metadata.json files has an inconsistent convention for name_pretty: for some APIs , it ends with the acronym API, leading to stutter in summary_overview.md ("Foo API API"; first noticed in #13100), and for some APIs it does not.

We need to:

1. establish a clear convention for the name_pretty format and for synthtool templates;
2. enforce the convention in google-cloud-python (e.g. have automation that adds/removes the appropriate string suffix, so that, for example, name_pretty doesn't end in "API"); and
3. enforce the convention in synthtool, making sure the templates generate files that read smoothly.