chore(bigframes): clean up the the AI package

[sycai]

The contents of the change are:

• Skipped all the doctests for the AI package.
• Fixed Sphinx format error: single backticks are replaced with double backticks.
• Removed parameter verification to keep the client lean.
• Removed pre-GA notes as these functions are all GA'ed
• Fixed format of some other files .

[gemini-code-assist]

Code Review

This pull request updates the documentation and refactors validation logic within the AI operations module. Key changes include adding doctest skip markers to examples, standardizing docstring formatting with double backticks for literals, and removing 'Pre-GA' notice blocks. Additionally, manual argument validation for the embed and similarity functions has been removed along with their corresponding unit tests, and imports have been reorganized across several files. I have no feedback to provide as there were no review comments to assess.