fix broken links (#13981)

Author: PhilKang0704

docs/extensibility/adding-an-lsp-extension.md

@@ -368,7 +368,7 @@ When tracing is turned on the content is written to a file in the *%temp%\Visual
 
 ### Custom messages
 
-There are APIs in place to facilitate passing messages to and receiving messages from the language server that are not part of the standard Language Server Protocol. To handle custom messages, implement [ILanguageClientCustomMessage2](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2) interface in your language client class. [VS-StreamJsonRpc](https://github.com/Microsoft/vs-streamjsonrpc/blob/main/doc/index.md) library is used to transmit custom messages between your language client and language server. Since your LSP language client extension is just like any other Visual Studio extension, you can decide to add additional features (that are not supported by the LSP) to Visual Studio (using other Visual Studio APIs) in your extension through custom messages.
+There are APIs in place to facilitate passing messages to and receiving messages from the language server that are not part of the standard Language Server Protocol. To handle custom messages, implement [ILanguageClientCustomMessage2](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2) interface in your language client class. [VS-StreamJsonRpc](https://github.com/microsoft/vs-streamjsonrpc/blob/main/docfx/index.md) library is used to transmit custom messages between your language client and language server. Since your LSP language client extension is just like any other Visual Studio extension, you can decide to add additional features (that are not supported by the LSP) to Visual Studio (using other Visual Studio APIs) in your extension through custom messages.
 
 #### Receive custom messages
 
@@ -409,7 +409,7 @@ internal class MockCustomLanguageClient : MockLanguageClient, ILanguageClientCus
 
 #### Send custom messages
 
-To send custom messages to the language server, implement the [AttachForCustomMessageAsync](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2.attachforcustommessageasync) method on [ILanguageClientCustomMessage2](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2). This method is invoked when your language server is started and ready to receive messages. A [JsonRpc](https://github.com/Microsoft/vs-streamjsonrpc/blob/main/src/StreamJsonRpc/JsonRpc.cs) object is passed as a parameter, which you can then keep to send messages to the language server using [VS-StreamJsonRpc](https://github.com/Microsoft/vs-streamjsonrpc/blob/main/doc/index.md) APIs. Example below:
+To send custom messages to the language server, implement the [AttachForCustomMessageAsync](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2.attachforcustommessageasync) method on [ILanguageClientCustomMessage2](/dotnet/api/microsoft.visualstudio.languageserver.client.ilanguageclientcustommessage2). This method is invoked when your language server is started and ready to receive messages. A [JsonRpc](https://github.com/Microsoft/vs-streamjsonrpc/blob/main/src/StreamJsonRpc/JsonRpc.cs) object is passed as a parameter, which you can then keep to send messages to the language server using [VS-StreamJsonRpc](https://github.com/microsoft/vs-streamjsonrpc/blob/main/docfx/index.md) APIs. Example below:
 
 ```csharp
 internal class MockCustomLanguageClient : MockLanguageClient, ILanguageClientCustomMessage2