Document ISO 8601 duration parsing in `pendulum.parse()`

[aeroyorch]

• I have searched the issues of this repo and believe that this is not a duplicate.

Issue

Hi, I think it would be helpful to document that pendulum.parse() supports ISO 8601 duration strings and provide examples/edge cases.

I could not find this clearly mentioned in the parsing or duration docs, and a short example would make it easier to discover.