Simplify, align and clarify related processes

[duncandewhurst]

• Closes Simplify, align and clarify related processes #1520

[jpmckinney]

(2) I think the descriptions in the beginning should also more emphasize the two important, popular and possibly less intuitive use cases: connecting planning and contracting processes and connecting different stages of a multi-stage procedure.

Furthermore, the wording of all the texts shold be more aligned.

I updated relatedProcesses and RelatedProcess to match each other and to mention both of these use cases.

The text from https://standard.open-contracting.org/staging/1.2-dev/en/schema/codelists/#related-process now just copies the text for RelatedProcess, as the original text adds no additional information compared to the new text.

[jpmckinney]

I forgot to review the codes in my previous review.

[duncandewhurst]

For some reason, I had deleted the old codes, rather than deprecating them. I've now reinstated the codes and added the deprecation information.

[duncandewhurst]

@jpmckinney just checking that you saw the review request on this PR. Thanks!

[jpmckinney]

See also my comment: #1645 (comment)