Set CMake code block language to txt as a fallback

[Rageking8]

Since code block language cmake is not supported in this repository (#5808 (review)), we have mainly 3 options:

1. Keep it empty
   • No syntax highlighting
   • Might be seen as "missing" or "incomplete"
2. Put txt
   • No syntax highlighting
   • Matches the .txt extension of CMakeLists.txt
   • Could be mistaken as a normal text file/output rather than CMake
3. Put some other language
   • Potentially plausible syntax highlighting
   • Might be confusing or misleading

Overall, I am in favor of option 2.

[prmerger-automator]

@Rageking8 : Thanks for your contribution! The author(s) and reviewer(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 5cd8542:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/build/cmake-presets-vs.md	✅Succeeded
docs/build/cmake-projects-in-visual-studio.md	✅Succeeded

For more details, please refer to the build report.

[TylerMSFT]

I think you make a good choice here.

[TylerMSFT]

#sign-off