Update preprocessor directives documentation (#48740)

* Update preprocessor directives documentation

Updated the date for the preprocessor directives documentation and added new information about the `#:project` token.

* Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>

Author: IEvangelist

docs/csharp/language-reference/preprocessor-directives.md

@@ -1,7 +1,7 @@
 ---
 description: "Learn the different C# preprocessor directives that control conditional compilation, warnings, nullable analysis, and more"
 title: "Preprocessor directives"
-ms.date: 06/19/2025
+ms.date: 09/24/2025
 f1_keywords:
   - "cs.preprocessor"
   - "#nullable"
@@ -109,6 +109,20 @@ The `#:` directives that are used in file-based apps include:
   ```xml
   <PackageReference Include="System.CommandLine" Version="2.0.0-*">
   ```
+
+- `#:project`:
+
+  Instances of `#:project` are translated into `ProjectReference` elements to include the project with the specified path to the project. For example:
+
+  ```csharp
+  #:project ../Path/To.Example.csproj
+  ```
+
+  The preceding preprocessor token is translated into:
+
+  ```xml
+  <ProjectReference Include="../Path/To.Example.csproj" />
+  ```
 
 Tools can add new tokens following the `#:` convention.