Enable Github "Discussions"

[tgross35]

This repo gets a lot of issues that are really more of questions. Examples:

• Benchmarking With One function result as Input  #766
• "cargo bench -- <filter>" does not actually filter the benchmarks #763
• Bench with huge input #740
• Is there a guide for migrating from 0.2.* to 0.4.*? #671

Enabling GH's Discussions tab would give a better place to ask questions without cluttering up the issues list. Those existing questions could probably be converted to discussions.

@lemmih I am not sure if you have repo managment access, @bheisler may have to do this.