firestore: fix doc link (#2539)
firestore: fix doc link (#2539)

Fix the link so that it does not 404.
Also change from homepage and example to
product and library docs to match other APIs.

Fixes #2538.
vam-google/google-cloud-java,FirebasePrivate/google-cloud-java,mbrukman/gcloud-java,mbrukman/gcloud-java,mbrukman/gcloud-java,mbrukman/gcloud-java,vam-google/google-cloud-java,vam-google/google-cloud-java,FirebasePrivate/google-cloud-java,FirebasePrivate/google-cloud-java,vam-google/google-cloud-java,FirebasePrivate/google-cloud-java,FirebasePrivate/google-cloud-java,vam-google/google-cloud-java,vam-google/google-cloud-java