docs: add code sample snippets for welcome page documentation

[shuoweil]

Adds sample code snippets with region tags (# [START ...] / # [END ...]) for BigQuery DataFrames documentation welcome page (python-libraries.md).

Note here, we need to place all imports inside testcase functions for the following reasons:

1. Doc Ingestion Boundaries: Documentation generation tools only extract code between # [START] and # [END] tags. Any top-of-file imports get stripped out.
2. Copy-Paste Runnable: Placing imports inside the function ensures the rendered code snippet on cloud.google.com includes the import statements, allowing users to copy and run it without hitting a NameError.

Fixes #<522845525> 🦕

[snippet-bot]

No region tags are edited in this PR.

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[gemini-code-assist]

Code Review

This pull request adds a new test file, packages/bigframes/samples/snippets/samples_test.py, which includes several test cases for BigQuery BigFrames snippets. The reviewer identified that these tests will fail in CI environments due to the use of placeholder project IDs and table names, and provided actionable feedback to mock the API calls using unittest.mock to ensure the tests are runnable.

[shuoweil]

Add the code sample using another branch: GoogleCloudPlatform/python-docs-samples#14429