Adjust documentation creation to new repo structure (#3274)

1) Add `utilities/stage_sites.py` (probably temporary solution).
2) Remove `utilities/create_site.sh` and `utilities/stage_release.sh` scripts. Other `.sh` releasing scripts will be removed soon. `RELEASING.md` was not update to reflect the changes. The readme will be updated after first successfull release with new structure (the only way to have an accurate releasing reame is to make the release first and record the steps).
3) Remove `.settings` folder. It is an eclipse-specific folder which hasn't been updated for 2 years. Nobody uses eclipse in our team, plus we should keep repo IDE-independent.
4) Move documentaiton look & feel files (.css, .html and .js files) to proper location (to `google-cloud-clients` from root).
5) Remove `google-cloud` maven dependency sample from landing page (since `google-cloud` metapackage was removed)
5) Several minor documentaiton-related fixes.

google-api-grpc/pom.xml

@@ -80,7 +80,7 @@
     <site>
       <id>github-pages-site</id>
       <name>Deployment through GitHub's site deployment plugin</name>
-      <url>site</url>
+      <url>site/google-api-grpc</url>
     </site>
   </distributionManagement>
   <licenses>

google-cloud-clients/pom.xml

@@ -153,7 +153,7 @@
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
     <github.global.server>github</github.global.server>
-    <site.installationModule>google-cloud</site.installationModule>
+    <site.installationModule>google-cloud-clients</site.installationModule>
     <bom.version>0.47.1-alpha-SNAPSHOT</bom.version><!-- {x-version-update:google-cloud-bom:current} -->
     <api-client.version>1.23.0</api-client.version>
     <gax.version>1.25.0</gax.version>