[Internal] Documentation: Adds SDK and Gateway Sequence Diagram.

docs/sdk_rg_seq_diagram.md

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+# Sequence Diagram for .NET v3 SDK and Gateway Interactions
+
+## Table of Contents
+
+* [Scope.](#scope)
+* [Sequence Diagram to Read Database and Collection Information.](#sequence-diagram-to-read-database-and-collection-information)
+* [Sequence Diagram to Read PkRange and Address Information.](#sequence-diagram-to-read-pkRange-and-address-information)
+
+## Scope
+
+The scope of this sequence diagram is to capture the compute/ routing gateway interactions with the `CosmosClient` configured in `Direct` mode.
+
+## Sequence Diagram to Read Database and Collection Information.
+
+```mermaid
+sequenceDiagram
+    participant A as Cosmos .NET <br> v3 SDK
+    participant B as GatewayAccountReader <br> [v3 Code]
+    participant C as ClientCollectionCache <br> [v3 Code]    
+    participant D as PartitionKeyRangeCache <br> [v3 Code]    
+    participant E as GatewayAddressCache <br> [v3 Code]    
+    participant F as GatewayStoreModel <br> [v3 Code]
+    participant G as CosmosHttpClient <br> [v3 Code]
+    participant H as Routing Gateway <br> [Deployed on each Region]
+    A->>B: Get Database <br> Account  
+    B->>G: HttpRequestMessage <br> (ResourceType = DatabaseAccount)
+    G->>H: HttpRequestMessage <br> (ResourceType = DatabaseAccount)
+    H-->>G: DatabaseAccount <br> Metadata Properties
+    G-->>B: DatabaseAccount <br> Metadata Properties
+    B-->>A: DatabaseAccount <br> Metadata Properties
+    A->>C: Read Collection <br> Information
+    C->>F: DocumentServiceRequest <br> (OperationType = Read) <br> (ResourceType = Collection)
+    F->>G: DocumentServiceRequest
+    G->>H: HttpRequestMessage <br> (ResourceType = Collection)
+    H-->>G: Collection <br> Metadata Information
+    G-->>F: Collection <br> Metadata Information
+    F-->>C: Collection <br> Metadata Information
+    C-->>A: Collection <br> Metadata Information   
+```
+
+## Sequence Diagram to Read PkRange and Address Information.
+
+```mermaid
+sequenceDiagram
+    participant A as Cosmos .NET <br> v3 SDK
+    participant B as GatewayAccountReader <br> [v3 Code]
+    participant C as ClientCollectionCache <br> [v3 Code]    
+    participant D as PartitionKeyRangeCache <br> [v3 Code]    
+    participant E as GatewayAddressCache <br> [v3 Code]    
+    participant F as GatewayStoreModel <br> [v3 Code]
+    participant G as CosmosHttpClient <br> [v3 Code]
+    participant H as Routing Gateway <br> [Deployed on each Region]
+    A->>D: Get PkRange Call
+    D->>F: DocumentServiceRequest <br> (OperationType = ReadFeed) <br> (ResourceType = PartitionKeyRange)
+    F->>G: DocumentServiceRequest
+    G->>H: HttpRequestMessage <br> (ResourceType = PartitionKeyRange)
+    H-->>G: PartitionKeyRange <br> Metadata Information
+    G-->>F: PartitionKeyRange <br> Metadata Information 
+    F-->>C: PartitionKeyRange <br> Metadata Information  
+    C-->>A: PartitionKeyRange <br> Metadata Information
+    A->>E: Get Address <br> Information
+    E->>F: DocumentServiceRequest <br> (OperationType = Read) <br> (ResourceType = Address)
+    F->>G: DocumentServiceRequest
+    G->>H: HttpRequestMessage <br> (ResourceType = Address)
+    H-->>G: Partition Address <br> Metadata Information
+    G-->>F: Partition Address <br> Metadata Information 
+    F-->>E: Partition Address <br> Metadata Information  
+    E-->>A: Partition Address <br> Metadata Information      
+```