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

googleapis/python-bigquery,googleapis/python-bigquery