Fix Cloud Function invoke operator docs link

[EltonChang1]

Add a dedicated CloudFunctionInvokeFunctionOperator section to the Google Cloud Functions operator docs and point the operator docstring's seealso link at that section instead of the deploy operator guide.

This addresses the docs bug called out in #68908 where the API docs for CloudFunctionInvokeFunctionOperator linked to a how-to guide for a different operator.

related: #68908

Validation performed:

• ruff format --check providers/google/src/airflow/providers/google/cloud/operators/functions.py
• ruff check providers/google/src/airflow/providers/google/cloud/operators/functions.py
• git diff --check

Limitations:

• Full docs build was not run locally; uv and breeze are not installed in this environment.

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Was generative AI tooling used to co-author this PR?

• Yes (OpenAI Codex/ChatGPT)

Generated-by: OpenAI Codex/ChatGPT following the guidelines

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