[backport#15368] Add checksums to descriptors.md

Summary:
https://github.com/bitcoin/bitcoin/pull/15368/commits/fd637be8d21a606e98c037b40b268c4a1fae2244

Depends on D6602

Concludes backport of Core PR15368

Test Plan:

none

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6609