feat(brain): add endpoint to return context to question

[ZongZiWang]

Description

• Add endpoint for getting context to a question from brain.
• The endpoint can be used by 3rd party app that does not want to get LLM answer, but instead just want to get the vector store search result for a query.
  • Example: RealChar using it here quivr-integration: Use brain question context endpoint Shaunwei/RealChar#406

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented hard-to-understand areas
• New and existing unit tests pass locally with my changes

Screenshots (if appropriate):

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 27, 2023 4:58am

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[StanGirard]

Love it ! It is going in production Monday if everything is fine with the other features.