docs: Rename pyhf and funcX requirements file for clarity

Following https://github.com/matthewfeickert/distributed-inference-with-pyhf-and-funcX/issues/5#issuecomment-841695178, rename requirements.txt to core-requirements.txt to indicate that it is not fully inclusive of dependencies, as a requirements.txt file generally is. Additionally, update this everywhere and try to give better descriptions in the docs.

* Rename requirements.txt to core-requirements.txt
   - Try to indicate that it is not fully inclusive of all dependencies
* Update README to note that core-requirements.txt provides pyhf and funcX dependencies and additional backends have their own requirements.txt files

matthewfeickert / distributed-inference-with-pyhf-and-funcX

docs: Rename pyhf and funcX requirements file for clarity #10