Documentation of the coverage validation in a separate rst or markdown file

[vinay0000]

Document detailed description of the coverage validation with STK done in the following script:
https://github.com/EarthObservationSimulator/orbitpy-revised/blob/98ed20e4997d88f602dd47abe24ef44fe4a0e2f3/slowtests/test_validation.py

Similar documentation exists for the old OrbitPy below:
https://github.com/EarthObservationSimulator/orbitpy/blob/master/docs/validation.rst
Difference with the old OrbitPy validation is that the satellite input trajectories in the (new) OrbitPy are imported from STK data, ensuring that both (new) OrbitPy and STK use identical trajectories for their respective coverage calculations.