API documentation - add best practices for fetching images from the events endpoint

[carrythebanner]

Update the Events endpoint documentation in docs/CALENDAR_API.md with recommendations for image fetching.

In short:

• Don't, unless you need to, especially for range requests
• Images must be fetched separate from the main events request; each event object provides a URL to its image, if it has one
• Clients should be able to handle image request failures (e.g. 404, 429, 5xx)
• Caching is encouraged; check the file's sequence ID to see if it's been updated since it was last fetched (and/or modified date, if that's available; see #578)

[ionous]

those are good rules. for the last one on that list, web clients requesting through http will automatically use browser caching rules and the rules in shift.conf; which is set to max ( 10 years ish )

https://github.com/shift-org/shift-docs/blob/5372f850ab0d13750b95d9f90fd596dd485cb53d/services/nginx/conf.d/shift.conf#L61

for iphone or other clients, if they use something like https://developer.apple.com/documentation/foundation/nsurlrequest it should ( in theory ) automatically cache according to the same rules.