[9.18] new: dev: generate changelog from git log

Use a single source of truth, the git log, to generate the list of CHANGES. Use the .rst format and include it in the ARM for a quick reference with proper gitlab links to issues and merge requests.

Closes #75

Backport of MR !9152

Merge branch 'nicki/add-gitchangelog-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9181