docs(epsilla): add epsilla integration guide (#1838)

Signed-off-by: Tony Yang <[email protected]>

Author: tonyyanga

docs/API_reference/doc_index/backends/epsilla.md

@@ -0,0 +1,3 @@
+# EpsillaDocumentIndex
+
+::: docarray.index.backends.epsilla.EpsillaDocumentIndex

docs/user_guide/storing/docindex.md

@@ -37,6 +37,7 @@ Currently, DocArray supports the following vector databases:
 - [Weaviate](https://weaviate.io/)  |  [Docs](index_weaviate.md)
 - [Qdrant](https://qdrant.tech/)  |  [Docs](index_qdrant.md)
 - [Elasticsearch](https://www.elastic.co/elasticsearch/) v7 and v8  |  [Docs](index_elastic.md)
+- [Epsilla](https://epsilla.com/)  |  [Docs](index_epsilla.md)
 - [Redis](https://redis.com/)  |  [Docs](index_redis.md)
 - [Milvus](https://milvus.io/)  |  [Docs](index_milvus.md)
 - [HNSWlib](https://github.com/nmslib/hnswlib)  |  [Docs](index_hnswlib.md)
@@ -67,11 +68,13 @@ from docarray.index import InMemoryExactNNIndex
 from docarray.typing import NdArray
 import numpy as np
 
+
 class MyDoc(BaseDoc):
     title: str
     price: int
     embedding: NdArray[128]
 
+
 docs = DocList[MyDoc](
     MyDoc(title=f"title #{i}", price=i, embedding=np.random.rand(128))
     for i in range(10)
@@ -120,6 +123,7 @@ To learn more and get the most out of `DocArray`, take a look at the detailed gu
 - [Weaviate](https://weaviate.io/)  |  [Docs](index_weaviate.md)
 - [Qdrant](https://qdrant.tech/)  |  [Docs](index_qdrant.md)
 - [Elasticsearch](https://www.elastic.co/elasticsearch/) v7 and v8  |  [Docs](index_elastic.md)
+- [Epsilla](https://epsilla.com/)  |  [Docs](index_epsilla.md)
 - [Redis](https://redis.com/)  |  [Docs](index_redis.md)
 - [Milvus](https://milvus.io/)  |  [Docs](index_milvus.md)
 - [HNSWlib](https://github.com/nmslib/hnswlib)  |  [Docs](index_hnswlib.md)