Add documentation for Globus Flows

**Is your feature request related to a problem? Please describe.**
There is no documentation about how to use funcX within a flow.

**Describe the solution you'd like**
Add a readthedocs page explaining the funcX AP interface with an example of using funcX within a flow.

**Describe alternatives you've considered**
NA

**Additional context**
NA

Start by reviewing the existing ReadTheDocs structure and the funcX API materials, then identify where a Globus Flows page belongs. Document how to use the funcX API within a flow and include a working example. Done means the new page is discoverable in the documentation and explains the requested workflow clearly.

Written by the indexing model from the issue text.