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-bigtable/README.rst

@@ -1,8 +1,7 @@
-Python Client for Google Cloud Bigtable (`Beta`_)
-==================================================
-
-|pypi| |versions|
+Python Client for Google Cloud Bigtable
+=======================================
 
+|beta| |pypi| |versions|
 
 `Google Cloud Bigtable`_ is Google's NoSQL Big Data database service. It's the
 same database that powers many core Google services, including Search,
@@ -11,7 +10,8 @@ Analytics, Maps, and Gmail.
 - `Client Library Documentation`_
 - `Product Documentation`_
 
-.. _Beta: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/README.rst
+.. |beta| image:: https://img.shields.io/badge/support-beta-silver.svg
+   :target: https://github.com/googleapis/google-cloud-python/blob/master/README.rst#beta-support
 .. |pypi| image:: https://img.shields.io/pypi/v/google-cloud-bigtable.svg
    :target: https://pypi.org/project/google-cloud-bigtable/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-bigtable.svg