Add NuGet guides and move Sass files out of wwwroot

[gumbarros]

Description

• Add dedicated README files for the bootstrap and bootstrap.sass NuGet packages.
• Document setup instructions for both .NET Framework and SDK-style projects.
• Move Sass files from contentFiles/any/any/wwwroot/scss to contentFiles/any/any/Styles.
• Add CI checks for NuGet package contents and .NET publishing.

Motivation and context

The NuGet packages previously showed only installation commands and did not explain how to use the included assets.

The Sass package also published source files under wwwroot, making the .scss files publicly accessible in ASP.NET Core applications at the browser.

Type of changes

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that changes existing functionality)

This breaking change is very worth, nobody from ASP.NET Core world expects a .scss file in wwwroot, wwwroot are only meant for browser files. Even for v5 I think its worth because its a bug.

Checklist

• I have read the contributing guidelines.
• My code follows the project's code style (npm run lint).
• My change introduces documentation changes.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Live preview

https://deploy-preview-42788--twbs-bootstrap.netlify.app/

Related issues

Fixes #42742