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