docs(discoveryengine): Add Search Tuning Samples

[holtskinner]

Description

https://cloud.google.com/generative-ai-app-builder/docs/tune-search

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• discoveryengine/train_custom_model_sample.py:16, tag genappbuilder_train_custom_model
• discoveryengine/update_serving_config_sample.py:16, tag genappbuilder_update_serving_config

---

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

[msampathkumar]

LGTM. Approved with a nit.

[msampathkumar]

@holtskinner - please check the comments before merging the PR.