chore(docs): Add final newline to markdown files (#19645)

Several documentation files were missing a final newline character.
This change adds them to adhere to POSIX conventions and improve
compatibility with various text-processing tools. It also helps
to create cleaner diffs in the future.

docs/api-compat.md

@@ -29,4 +29,4 @@ API changes are validated against a **baseline version**—the reference point f
 
 ## Additional Resources
 
-- [API Validation Tool Implementation](https://github.com/AvaloniaUI/Avalonia/pull/12072) - Pull request that introduced this feature
+- [API Validation Tool Implementation](https://github.com/AvaloniaUI/Avalonia/pull/12072) - Pull request that introduced this feature

docs/build.md

@@ -108,4 +108,4 @@ It is possible to run some .NET Framework samples and tests using .NET Framework
 
 ## Building Avalonia into a local NuGet cache
 
-See [Building Local NuGet Packages](nuget.md)
+See [Building Local NuGet Packages](nuget.md)

docs/porting-code-from-3rd-party-sources.md

@@ -9,4 +9,4 @@ When porting code or adapting code from other projects with a MIT compatible lic
 // Licensed to The Avalonia Project under MIT License, courtesy of The .NET Foundation.
 ```
 
-If the file is a port of a specific commit of file from a 3rd party source, then consider including a permalink to the source file.
+If the file is a port of a specific commit of file from a 3rd party source, then consider including a permalink to the source file.