chore: complete bigframes migration

[jskeet]

• Reformat with nox
• Enable generation with librarian
• Regenerate with librarian

[gemini-code-assist]

Code Review

This pull request transitions the bigframes library to a stable release level, updates its metadata, and adds a detailed README.rst documentation file. Review feedback suggests consolidating split imports from the same module in loader.py for better idiomatic consistency and correcting a typo in the documentation.

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packages/bigframes/bigframes/session/loader.py (55-60)

The imports from google.cloud.bigquery_storage_v1 have been split into two separate blocks. It is more idiomatic in Python to group imports from the same module together in a single block.

from google.cloud.bigquery_storage_v1 import (
    types as bq_storage_types,
    writer as bq_storage_writer,
)

References

1. PEP-8 recommends grouping imports from the same module together to improve readability and organization. ^(link)

packages/bigframes/docs/README.rst (40)

There is a typo and redundant word in this line: "environment enviroment".

   for your local development environment.