readme enhancement for nuget package

[WeihanLi]

**Closes #148 **

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Add the NuGet package info and links, thanks for @Nirmal4G 's help

Preview: https://github.com/WeihanLi/dotnet-CommunityTookit/blob/readme-nuget-package-enhancement/README.md#-what-does-this-repo-contain

[Nirmal4G]

@Sergio0694 @michael-hawker Also, the docs.microsoft.com has been changed to learn.microsoft.com. Do we need to update it here or in a separate PR?

[Nirmal4G]

🎊

[Sergio0694]

Thank you! 🙂

[Sergio0694]

@Nirmal4G The docs have redirects, so we can update them later on if we want. We're using docs everywhere anyway 😄

[Nirmal4G]

I was aware of that. I only discovered through this PR. Just notifying the project maintainers. That's all. 😇