Update schema reference for Contract.relatedProcesses

[duncandewhurst]

The deprecated field is still mentioned in the Markdown content below the jsonschema directive: https://standard.open-contracting.org/staging/1.2-dev/en/schema/reference/#relatedprocess

[duncandewhurst]

Per #1714 (comment), also need to explain that contracting processes should be linked to planning processes using the 'planning' code.