Remove screenshots of UI and spreadsheets

[emlys]

Some model documentation includes screenshots of the InVEST UI and of example spreadsheets which are prone to becoming outdated. I don't think we need to have screenshots of the UI at all. Example spreadsheets should be stored as CSVs. It would be extra cool if we could link to the external sample data rather than duplicating it here.
Noticed this in the following UG pages:

• fisheries
• habitat quality
• hra
• invest api
• marine fish
• ndr
• wave energy

[dcdenu4]

Hey @emlys , I'm onboard with your thoughts here about UI screenshots and spreadsheets. I just wanted to mention that for HQ it looks like the spreadsheet like screenshot is a figure from a publication, so probably fine to leave alone. For wave energy, those spreadsheet screenshots could be made into markdown tables.... but that seems like unnecessary work. Just my thoughts on the models I consider myself responsible for!