Add docs for APIs like AddPoliciesEx()

[Eric Luo]

Docs for PR: casbin/casbin#1196

1. Add AddEx API section after: https://casbin.org/docs/api-overview#add-delete-update-api
2. Add all new API functions to reference: https://casbin.org/docs/management-api

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[Eric Luo]

@PokIsemaine