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/create-named-vector/dense/_description.md

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+Create a new dense named vector on an existing collection. Only the immutable vector-space properties are required: size and distance. Storage type, index, and quantization can be configured separately afterward.

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

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+```http
+PUT /collections/{collection_name}/vectors/{vector_name}
+{
+    "dense": {
+        "size": 256,
+        "distance": "Cosine"
+    }
+}
+```

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

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+Create a new sparse named vector on an existing collection. Only the immutable vector-space properties are required. Storage type, index, and quantization can be configured separately afterward.

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

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+```http
+PUT /collections/{collection_name}/vectors/{vector_name}
+{
+    "sparse": {
+        "modifier": "Idf"
+    }
+}
+```