Document implement a custom endpoint

[danielballan]

Include:

• Short explanation of why Tiled resists adopting "data processing" scope
• Mention alternative of running a fully separate microservice
• Present adding a custom endpoint, as a plugin, as a compromise option—easier than deploying a separate service, but still keep heavy and/or domain-specific processing out of Tiled core

Then show an example that does some nominal data processing task, like adding 1 to a dataset or something.