Add Kapa search

[akeller]

Description

Based on user behavior with Kapa, I can see people using the Ask AI agent as a search rather than a conversational bot. Shouting terms at it and getting a generated response rather than a list of resources is frustrating, although I don't have data to specifically backup that sentiment.

This introduces a toggle on the modal itself, with search powered by Kapa.

This makes no changes to the search experience in the top nav, still powered by Algolia Docsearch.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.

• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer.

• My changes require a docs infrastructure review.