chore(ask-ai): document Ask AI search banner results target

[Mcourt]

Summary

Documents the new ASK_AI_SEARCH_BANNER_RESULTS target, which renders customizations inside the Ask AI answer (collapsible area), and clarifies how it differs from the existing ASK_AI_SEARCH_BANNER target.

• Adds targets.ASK_AI_SEARCH_BANNER_RESULTS to the targets table in the API reference.
• Adds a "Ask AI Search banner results" capability section (compatibility table + notes).
• Clarifies that ASK_AI_SEARCH_BANNER customizations render outside the answer block and stay visible whether the answer is collapsed or expanded.
• Documents that customizations on both targets stay hidden until the answer has finished streaming, so components such as extensions can pre-load their data.
• Adds a use case "Display a message at the end of the Ask AI answer" with a code sample, and notes that both targets can be used together.

Changes

• docs/javascript/api.md — targets table entry
• docs/javascript/capabilities.md — new target section + updated banner notes
• docs/javascript/use-cases.md — new use case + updated banner use case

Testing

Documentation-only change — no manual testing required. Rendered links to ./capabilities#ask-ai-search-banner-results and ./use-cases#display-a-message-at-the-end-of-the-ask-ai-answer match the new anchors.

Non-regression

Existing Ask AI banner documentation is only reworded, no target or snippet removed.