STIPS 2.3.1 bug fixes with the documentation updates

[eunkyuh]

Several bug fixes include:

• Proper unit conversion for the Pandeia background
• Logic to replace SCAs with WFIs
• Distinguished in the output header for the input X & Y positions and the ePSF X & Y positions

Also updating the hard-coded zeropoint values based on the Roman Technical Repo

Documentation updates include

• Remaining mention of WebbPSF to replace with STPSF
• Removal of 'oversample' key in the example that no longer works in the newer versions of STIPS
• psf_grid_size fixed to 3x3

Installation yml updates include pandeia.engine requirements to be 2025.9 or higher to use the latest engine.