Update graph doc for installation (mem0ai#1959)

Author: Dev-Khant

docs/open-source/graph_memory/overview.mdx

@@ -7,6 +7,16 @@ Mem0 now supports **Graph Memory**.
 With Graph Memory, users can now create and utilize complex relationships between pieces of information, allowing for more nuanced and context-aware responses. 
 This integration enables users to leverage the strengths of both vector-based and graph-based approaches, resulting in more accurate and comprehensive information retrieval and generation.
 
+## Installation
+
+To use Mem0 with Graph Memory support, install it using pip:
+
+```bash
+pip install mem0ai[graph]
+```
+
+This command installs Mem0 along with the necessary dependencies for graph functionality.
+
 Try Graph Memory on Google Colab.
 <a target="_blank" href="https://colab.research.google.com/drive/1PfIGVHnliIlG2v8cx0g45TF0US-jRPZ1?usp=sharing">
   <img src="https://colab.research.google.com/assets/colab-badge.svg" alt="Open In Colab"/>

mem0/memory/graph_memory.py

@@ -1,7 +1,14 @@
 import logging
 
-from langchain_community.graphs import Neo4jGraph
-from rank_bm25 import BM25Okapi
+try:
+    from langchain_community.graphs import Neo4jGraph
+except ImportError:
+    raise ImportError("langchain_community is not installed. Please install it using pip install langchain-community")
+
+try:
+    from rank_bm25 import BM25Okapi
+except ImportError:
+    raise ImportError("rank_bm25 is not installed. Please install it using pip install rank-bm25")
 
 from mem0.graphs.tools import (
     ADD_MEMORY_STRUCT_TOOL_GRAPH,