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.

packages/google-cloud-bigquery-datatransfer/README.rst

@@ -1,15 +1,16 @@
-Python Client for BigQuery Data Transfer API (`Alpha`_)
-=======================================================
+Python Client for BigQuery Data Transfer API
+============================================
 
-|pypi| |versions|
+|alpha| |pypi| |versions|
 
 The `BigQuery Data Transfer API`_ allows users to transfer data from partner
 SaaS applications to Google BigQuery on a scheduled, managed basis.
 
 - `Client Library Documentation`_
 - `Product Documentation`_
 
-.. _Alpha: https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/README.rst
+.. |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-bigquery-datatransfer.svg
  :target: https://pypi.org/project/google-cloud-bigquery-datatransfer/
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-bigquery-datatransfer.svg