docs: add embedding-driven semantic search guide and integrate with concepts

[benhalpern]

This PR introduces a new documentation guide for the newly added embedding-driven semantic search functionality in Forem's API.

Summary of Changes:

1. New Guide: Created docs/semantic-search.md documenting the new GET /api/articles/semantic_search and GET /api/concepts/search endpoints, highlighting parameters like threshold, explanations of distance vs. similarity, and curl request/response examples.
2. Concepts Guide Update: Updated docs/concepts.md to document the semantic concept search endpoint (GET /api/concepts/search) and cross-link to the new Semantic Search guide.
3. API Reference Index Link: Linked the new guide in docs/api.md for better discoverability.

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