Add missing RPC and REST documentation links where they exist

https://github.com/googleapis/google-cloud-java/pull/9913 was an initial seeding of REST/RPC documentation links.

Out of 162 repo-metadata.json files, 124 had valid product_documentation links (e.g. return http status = 200).

Out of those, these are the repos containing either (or both) an invalid rpc/rest documentation link based on the generic URI variation: https://paste.googleplex.com/5546150802227200

We should fill in the remainders where they exist (often due to unique directory naming, such as for java-apikeys (https://cloud.google.com/api-keys/docs/reference/rest), the automated seeding didn't work).

These links are used for the updated package overviews in Cloud RAD.