Enforce use of Enum items in method arguments

[fnesveda]

In some methods in the resource clients, we had the type of some arguments set to some Enum, but we actually worked with the arguments as if they were strings. This PR corrects it so that we're actually working with those arguments as Enum items, and updates the documentation of those arguments to point to those Enums.

The PR also adds the description of those Enums to the documentation, which required a small rework of how the formatting of the documentation works, adding to the mess that that is.

I've also renamed the _consts submodule to just consts, so that users can import those Enums from those consts like from apify_client.consts import ActorJobStatus and use the Enum items in the client methods.

[mhamas]

Thanks!