docs(api): document v5 GET memory (#1771)

Docs for supermemoryai/mono#3423: new `GET /ns/{namespace}/memories/{id}` with `include=related,documents` and `relatedLimit`.

apps/docs/snippets/api-v5-content-management.mdx

@@ -21,6 +21,17 @@ Pass `include` as a comma-separated list to return chunks, memories, or both. Ke
 
 `system.status` is one of `unknown`, `queued`, `extracting`, `chunking`, `embedding`, `indexing`, `done`, or `failed`.
 
+### Retrieve a memory and its history
+
+```bash v5
+GET /ns/{namespace}/memories/{id}?include=related,documents&relatedLimit=10
+```
+
+Returns one memory in the same shape as list memories. A memory from another namespace returns `404`; a forgotten memory, or one past its `forget_after`, is still returned with `isForgotten: true`.
+
+- `include=related` adds `included.related.parents` (earlier versions), `children` (newer versions), and `siblings` (memories connected by `extends` or `derives`). Each list walks outward from the memory until it holds `relatedLimit` items (default `10`, maximum `100`). Forgotten memories and memories in other namespaces are left out.
+- `include=documents` adds `included.document`, the most recently updated source document. With both values, every related memory also carries its own `document`.
+
 ### List documents, chunks, or memories
 
 <CodeGroup>

apps/docs/v5/api-reference/content-management.mdx

@@ -10,6 +10,7 @@ Inspect the context stored in a namespace and remove information that should no
 | Operation | Purpose |
 | --- | --- |
 | `GET /ns/{namespace}/document/{id}` | Retrieve source content with its chunks or memories |
+| `GET /ns/{namespace}/memories/{id}` | Retrieve one memory with its history, connected memories, or source document |
 | `POST /ns/{namespace}/list/{type}` | Page through documents, chunks, or memories |
 | `DELETE /ns/{namespace}/document` | Remove one or many documents |
 | `DELETE /ns/{namespace}/memories` | Forget reviewed memories by exact ID |