update_changelog.py reads git log since the last version tag,
categorises commits by conventional-commit prefix, and prepends a
new ## [vX.Y.Z] entry to docs/changelog/index.md. changelog.yml
runs after build.yml succeeds and commits the result back to main
with [skip ci]. Supports dry_run and manual_version dispatch inputs.
Signed-off-by: nopeitsnothing <no@anonymousplanet.org>