Improve the Casbin.NET docs at Casbin website

[sagilio]

docs: https://github.com/casbin/casbin-website-v2/tree/master/docs

Task list:

• [ ] Complement lacked code part.
• [ ] Introduce Casbin.NET v2 new features.
• [ ] Add redis-adpter and redis-watcher.
• [ ] Add Casbin.NET v2 benchmarks.

[casbin-bot]

@sagilio @sociometry @AsakusaRinne