Docs: Specify URL endpoints for various uploaders

[IngwiePhoenix]

I went and read it up from the source code to figure out how to configure ShareX to allow me to shorten URLs - however, others might not be so savy :)

Hence, I'd like to request an Endpoints section in the docs, pointing out which URLs are used for what.

For those curious:

There's probably more methods I didn't spot yet, tho...

Thanks and kind regards!