chore: fix product documentation link (#589)

Author: Neenu1995

.repo-metadata.json

@@ -1,7 +1,7 @@
 {
   "name": "logging-logback",
   "name_pretty": "Cloud Logging Logback Appender",
-  "product_documentation": "https://cloud.google.com/logging-logback/docs/",
+  "product_documentation": "https://cloud.google.com/logging/docs/setup/java#logback_appender_for",
   "client_documentation": "https://cloud.google.com/java/docs/reference/google-cloud-logging-logback/latest/history",
   "issue_tracker": "https://issuetracker.google.com/savedsearches/559764",
   "release_level": "alpha",

README.md

@@ -226,7 +226,7 @@ Java 11 | [![Kokoro CI][kokoro-badge-image-5]][kokoro-badge-link-5]
 
 Java is a registered trademark of Oracle and/or its affiliates.
 
-[product-docs]: https://cloud.google.com/logging-logback/docs/
+[product-docs]: https://cloud.google.com/logging/docs/setup/java#logback_appender_for
 [javadocs]: https://cloud.google.com/java/docs/reference/google-cloud-logging-logback/latest/history
 [kokoro-badge-image-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-logging-logback/java7.svg
 [kokoro-badge-link-1]: http://storage.googleapis.com/cloud-devrel-public/java/badges/java-logging-logback/java7.html