give stub packages their own doc section (googleapis#2511)

Fixes googleapis#2476.

The list of packages created with command:
```shell
find -name '*.java' |
  xargs grep -hr 'package.*\.stub' |
  cut -c 9- |
  sort -u |
  tr ';' ':' |
  tr -d '\n'; echo
```

Author: pongad

pom.xml

@@ -1185,6 +1185,10 @@
                     <title>SPI packages</title>
                     <packages>com.google.cloud.spi*:com.google.cloud.bigquery.spi*:com.google.cloud.compute.spi*:com.google.cloud.datastore.spi*:com.google.cloud.dns.spi*:com.google.cloud.logging.spi*:com.google.cloud.resourcemanager.spi*:com.google.cloud.spanner.spi*:com.google.cloud.storage.spi*:com.google.cloud.translate.spi*</packages>
                   </group>
+                  <group>
+                    <title>Stub packages</title>
+                    <packages>com.google.cloud.dlp.v2beta1.stub:com.google.cloud.errorreporting.v1beta1.stub:com.google.cloud.firestore.v1beta1.stub:com.google.cloud.language.v1beta2.stub:com.google.cloud.language.v1.stub:com.google.cloud.logging.v2.stub:com.google.cloud.monitoring.v3.stub:com.google.cloud.pubsub.v1.stub:com.google.cloud.speech.v1beta1.stub:com.google.cloud.speech.v1.stub:com.google.cloud.trace.v1.stub:com.google.cloud.videointelligence.v1beta1.stub:com.google.cloud.videointelligence.v1beta2.stub:com.google.cloud.vision.v1.stub</packages>
+                  </group>
                 </groups>
                 <links>
                   <link>https://developers.google.com/protocol-buffers/docs/reference/java/</link>