Change Procedure rst docs to match what is in the protobuf

[cmungall]

On https://phenopacket-schema.readthedocs.io/en/latest/procedure.html

The Procedure element represents a clinical procedure performed on a subject. For example a surgical or diagnostic procedure such as a biopsy.

If the Procedure element is used, it must contain a code element, but only need contain the body_site element if needed.

This is hard to interpret. It only need to do it if needed? What is this saying?

The protobuf documentation is IMO much clearer:

// A clinical procedure performed on a subject. By preference a single concept to indicate both the procedure and the
// body site should be used. In cases where this is not possible, the body site should be indicated using a separate
// ontology class.

I suggest manually syncing the rst with the protobuf to reflect what is in the latter

(perhaps future versions phenopackets will use linkml which solves this manual linking problem...)