Add blocking robots file to default branch

[joel-hamill]

Description

What behavior do you want to change, why, how does your patch achieve the changes?
Adding custom robots.txt that tells search engines not to index, per the instructions.

Testing done

Describe the testing strategy. Unit and integration tests are expected for any behavior changes.

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")
• Do these changes have compatibility implications for rollback? If so, ensure that the ksql command version is bumped.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
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