job manager: rename "start_job" to "create_job"

[soxofaan]

The job manager API has this "start_job" callable:

openeo-python-client/openeo/extra/job_management/__init__.py

Lines 319 to 322 in 29a3276

	:param start_job:
	A callback which will be invoked with, amongst others,
	the row of the dataframe for which a job should be created and/or started.
	This callable should return a :py:class:`openeo.rest.job.BatchJob` object.

This callable allows the user to just create a job, or also start it.
The current name encourages the user to start it,
but I think we should actually recommend against that because that undermines the possibilities to handle the job starting in a smarter way (e.g. do the "start job" request in a worker thread for more efficient time management)

[soxofaan]

e.g. this is a stumbling block for #719

[soxofaan]

cc @HansVRP