Update Slack Sample READMEs

[mdrichardson]

The Slack sample READMEs (and docs - @JonathanFingold) need an update to how to set them up properly. This should be done after

https://github.com/microsoft/botbuilder-dotnet/issues/5115

and possibly after

https://github.com/microsoft/botbuilder-dotnet/issues/5519

Some issues:

• Sample recommends using /test command, when it should be /block and there's no instructions for setting up commands in Slack
• Token clarity
• New OAuth Tokens and needing to set scopes before installing app