Documentation points to missing setup.py

[ameraner]

Describe the bug
After #2763, the Satpy docs now point to the non-existing setup.py file in a couple of spots (see this search: https://github.com/search?q=repo%3Apytroll%2Fsatpy+setup.py&type=code ).

To Reproduce

Clicking on the hyperlink e.g. here: https://satpy.readthedocs.io/en/stable/install.html#pip-based-installation causes a 404 error.

Expected behavior
The mentions of setup.py in the docs should be removed and the according docs instructions updated.

[mraspaud]

I'm on it