Add semantic search for bills and testimony

[nesanders]

Suggestion Title
Leverage the new vector indexes for bills and testimony introduced in #2141 to enable semantic search on bill and testimony search pages.

Detailed Description
See https://firebase.google.com/docs/firestore/vector-search

Reason for the Suggestion
Would provide better performance than existing keyword-based search when the user does not know the exact legislative terminology associated with a policy.

Additional Context
N/A

Checklist

• I have checked if this enhancement is not already suggested
• I have explained the enhancement in detail