Edit & audit ESAP documentation

The ESAP wiki at https://git.astron.nl/astron-sdc/esap-api-gateway/-/wikis/ESAP-API-gateway-Overview has a lot of useful ESAP documentation. Plus, we have various readme files, text documents, other wikis, etc. However, this material is all pretty disjoint and hard to find — and some of it is probably outdated.

The goal of this ticket is to:

• Make a list of the available ESAP documentation (it isn't necessary to be comprehensive; “all the ESAP docs that are easy to find” is fine)
• Compile them into an organized collection in the main ESAP wiki (editing, combining, splitting, etc as necessary)
• Identify gaps and file tickets requesting material to fill them