Docs: normalize use of support level badges (#6159)

* Remove badges for deprecated umbrella 'google-cloud' package.

* Clarify support levels.

- Add explicit section to support linking from sub-package README badges.
- Move explanatory text for a support level above the list of packages
  at that level.

* Normalize use of support-level badges in READMEs.

- Note that 'error_reporting/README.rst' and 'monitoring/README.rst' are
  undergoing other edits;  they are left out here to avoid conflicts.

* Use 'General Avaialblity' for support level.

Fix linkx in related API READMEs.

* Fix links for alpha support in API READMEs.

* Fix links for beta support in API READMEs.

Author: tseaver

packages/google-cloud-dns/README.rst

@@ -1,14 +1,16 @@
 Python Client for Google Cloud DNS
 ==================================
 
-|pypi| |versions|
+|alpha| |pypi| |versions|
 
 The `Google Cloud DNS`_ API provides methods that you can use to
 manage DNS for your applications.
 
 - `Client Library Documentation`_
 - `Product Documentation`_
 
+.. |alpha| image:: https://img.shields.io/badge/support-alpha-orange.svg
+   :target: https://github.com/googleapis/google-cloud-python/blob/master/README.rst#alpha-support
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-dns.svg
    :target: https://pypi.org/project/google-cloud-dns/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-dns.svg