Docs for adding/removing named vectors (#2282)

* Initial commit

* Switch to title case for Collections page

* Move to Collections page

* One more clarification

* Update section title

qdrant-landing/content/documentation/headless/snippets/delete-named-vector/_description.md

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+Delete a named vector from an existing collection. This removes the vector schema and all associated data from every segment. Existing points lose this vector field; they are not otherwise affected.

qdrant-landing/content/documentation/headless/snippets/delete-named-vector/http.md

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+```http
+DELETE /collections/{collection_name}/vectors/{vector_name}
+```