Add Bigtable KNN notebook example

[cpcgoogle]

Add Bigtable KNN notebook example

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• [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigtable/issues/new/choose) before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea

Below not in scope since this is just a sample.

• [ ] Ensure the tests and linter pass
• [ ] Code coverage does not decrease (if any source code was changed)
• [ ] Appropriate docs were updated (if necessary)

Fixes # 🦕

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🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/

[daniel-sanche]

Sorry, we don't typically accept external samples

[daniel-sanche]

after discussing this offline, I believe we landed on having this live in a different repo. But let me know if you want to discuss it further