docs: add a sample notebook for vector search

[tswast]

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Fixes #<issue_number_goes_here> 🦕

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[tswast]

Notebook authored by @optidus originally posted at https://github.com/optidus/bigframes-for-vector-search/blob/main/Build%20a%20vector%20search%20application%20using%20BigFrames.ipynb for https://medium.com/google-cloud/build-a-vector-search-application-in-python-using-bigframes-c8baed3c4568

[chelsea-lin]

LGTM. Great notebook for vector search!