Add detail to pipeline file specification

[simonl2002]

The pipeline specification documentation only gives basic information about fields.

For instance the pipeline id field. Only has the following information:

Data Type: String
Required: Yes
Default: None
<SNIP>

If you look that the code you'll see that there are only certain characters allowed and there's a length limit.
https://github.com/ConduitIO/conduit/blob/cea8874345beed93ff10634ede6401a2c56a1afb/pkg/pipeline/service.go#L30-L36

We should just have this info in the spec so someone doesn't need to go to the code to look it up.