[docs] Describe which params can be set for ECS servicedef and taskdef

[t-kikuc]

What would you like to be added:

as title.

I'm not sure which page to explain them on. (here?: https://pipecd.dev/docs/user-guide/configuration-reference/)

Why is this needed:

• Some of the params are actually not used in servicedef/taskdef. (e.g. loadbalancers in servicedef)
• Users will get confused when configuring them but they're not working.

Details of the Params

all params in ECS:

• servicedef: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/service_definition_parameters.html
• taskdef: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html

configurable params in PipeCD:

• servicedef: https://github.com/pipe-cd/pipecd/blob/master/pkg/app/piped/platformprovider/ecs/client.go#L90-L107
• taskdef: https://github.com/pipe-cd/pipecd/blob/master/pkg/app/piped/platformprovider/ecs/client.go#L150-L163

[t-kikuc]

This is the same as #4961 and already solved.