[docs] Add example return values to all APIs in API reference pages

[hsluoyz]

Would be clearer if outputs are also shown along with the method calls in the ApI reference

Add example return values to all APIs in https://casbin.io/docs/management-api and https://casbin.io/docs/rbac-api

[casbin-bot]

@tangyang9464 @JalinWang @imp2002

[rajivharlalka]

@hsluoyz I would like to take up this issue, can you please assign it to me.

[hsluoyz]

@rajivharlalka we don't usually assign people but you can work on it

[rajivharlalka]

@hsluoyz Does adding an example return value only for Go(the default language selected) works, or does it need to be there for all languages

[hsluoyz]

Finally all will be added, can start from Go

[rajivharlalka]

@hsluoyz Had added some return values, could you confirm if the format works, and would add more in same format.