Improve PRDOC scheme for runtime changes

[bkchr]

@Ank4n created some nice list of changes in the following pr: #7424 (comment)

We should translate this list into the prdoc schema. Per pallet we should write down what got changed, removed or added. We should use the same categories "Event", "Call", "Error" "Storage", "Config".

This data could then be aggregated and shown in the release notes. Most of this data could probably also be automatically collected by diffing on the metadata or similar.

CC @the-right-joyce @EgorPopelyaev @mordamax @alvicsam

[jsdw]

We should use the same categories "Event", "Call", "Error" "Storage", "Config".

Don't forget "View Functions" :)