Enable Github Discussions

[timcassell]

Can the Discussions section be opened up? I feel some things I want to discuss aren't worthy of Issue status, and some discussions clog up issues and PRs (like in #2114 and #2111).

[AndreyAkinshin]

@timcassell thanks for the suggestion! The GitHub Discussions are enabled now:
https://github.com/dotnet/BenchmarkDotNet/discussions