Is there CiviContribute API documentation?
I do not know what you want to achieve exactly, but the API that I use is called Contribution. It creates a contribution in CiviCRM but it does not automatically make calls to PayPal or Stripe. 

And I am glad it does not because most NGO's in my corner of the world (Northern Europe) hardly use PayPal and have never heard of Stripe. They want all their payments processed with SEPA, which probably you never heard of.

Documentation on the API can be found here: https://docs.civicrm.org/dev/en/latest/api/