clarify how to get binaries if F5 results in error (#8788)

* clarify how to get binaries if F5 results in error

Documentation/Building the Extension.md

@@ -10,6 +10,14 @@ Prerequisite steps:
   * From a command line, run the following commands from the **Extension** folder in the root of the repository:
       * `yarn install` will install the dependencies needed to build the extension.
       * **(optional)** `yarn global add vsce` will install `vsce` globally to create a VSIX package that you can install.
-  * **(optional)** Set an environment variable `CPPTOOLS_DEV=1`.
-    * This enables the local developer workflow when testing the debugger, copying dependencies from the **node_modules** folder. Testing the language server does not require this step.
   * Open the **Extension** folder in Visual Studio Code and press F5. This will launch a VS Code Extension Host window and activate the TypeScript debugger. You can set breakpoints on the extension source code and debug your scenario.
+      * If, after pressing F5, you see the following error in the `[Extension Development Host]` window,
+      > Unable to start the C/C++ language server. IntelliSense features will be disabled. Error: Missing binary at .../vscode-cpptools/Extension/bin/cpptools
+      * Then, you can follow the instructions in this [comment in a discussion about building this extension locally](https://github.com/microsoft/vscode-cpptools/discussions/8745#discussioncomment-2091563).
+        > get the <package.version.number> binaries from installing the extension and then copying the binaries
+        1. To do this, install this extension from the Visual Studio Marketplace and find its location on your device. It might be in a directory like `\\wsl$\Ubuntu\home\hamir\.vscode-server\extensions\ms-vscode.cpptools-<package.version.number>`, for example.
+        2. Next, go to the `bin/` directory of the aforementioned directory, and drag-and-drop, or copy-and-paste, `cpptools` and `cpptools-srv` from `...\extensions\ms-vscode.cpptools-<package.version.number>\bin\` to this repository's `Extension\bin\` directory on your local device, so that `.../vscode-cpptools/Extension/bin/cpptools` and `.../vscode-cpptools/Extension/bin/cpptools-srv` both exist in your workspace.
+        3. The aforementioned warning should be gone, and Intellisense, which gives those squiggly red error lines, should now be present.
+        4. The `insiders` branch has binaries compatible with the latest Pre-Release version of the extension, and the `release` branch has binaries compatible with the latest Release version, but the `main` branch may have TypeScript changes that are incompatible with the published binaries, in which case, you'll need to create a branch off the `insiders` or `release` branches.
+
+      * Feel free to use [the Discussions tab of this repository](https://github.com/microsoft/vscode-cpptools/discussions) if you have any further questions on building this extension locally.