Move BigQuery Agent Analytics docs to observability directory and update the plugin doc to v2 (#1060)

* Move BigQuery Agent Analytics docs to plugins directory

* Enhance BigQuery Agent Analytics documentation

Updated the documentation for the BigQuery Agent Analytics plugin to clarify event logging configurations and added a code sample for setting up the plugin.

* Move BigQuery analytics to observability and update index link

* Remove default comment from max_content_length

* Update mkdocs.yml for BigQuery Analytics move

* Update BigQuery Agent Analytics configuration defaults

* Move BQ observabilty page in nav

* Fix markdown list formatting in IAM permissions

* Clarify ADK 1.21.0 version requirement

* Update BigQuery Agent Analytics documentation

* Strongly recommend latest ADK version

* Refine BigQuery Agent Analytics documentation

* Update bigquery-agent-analytics.md

* Remove deprecated documentation

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Co-authored-by: Kristopher Overholt <[email protected]>
Co-authored-by: Joe Fernandez <[email protected]>

Author: 3 people