Simplify and Clarify PICASO installation docs

[natashabatalha]

@rewaclarkbush

• Update python version suggestion
• Create designated "default" opacity file on zenodo that is 15k R with all the relevant molecules
• Update installation page to include creating conda environment before git setup
• Make a step that is "create default opacity" and have it just involve sticking the file into the folder (change code to auto detect a sqlite opacities database in the opacities folder (different from the continuum one)
• add instructions for creating a forked repository
• create a function to get the stellar docs
• make a suggestion for where to put stellar grid folder
• make the PYSYN_CDBS environment variables also go into the env_vars folder
• trapz issue with scipy make sure you have scipy not 1.15.1 (check where we are using trapz)