Updates doc to explain how to enable .razor files with TUnit

[johanbenschop]

Explains that TUnit only works with Razor files in reflection mode due to source generator limitations.

Provides instructions for setting up TUnit with .razor files using a .runsettings file and updating the .csproj.

Adds a note recommending a separate test project for bUnit tests to avoid performance issues.

Relates to #1661

Pull request description

PR meta checklist

• [X] Pull request is targeted at main branch for code
  or targeted at stable branch for documentation that is live on bunit.dev.
• [X] Pull request is linked to all related issues, if any.
• [X] I have read the CONTRIBUTING.md document.

Code PR specific checklist

• [ ] My code follows the code style of this project and AspNetCore coding guidelines.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [ ] I have updated the appropriate sub section in the CHANGELOG.md.
• [ ] I have added, updated or removed tests to according to my changes.
  • [ ] All tests passed.

[linkdotnet]

Looks good to me - thanks. Currently the PR is "in draft". Is there something "missing"?