The single CHANGELOG.md had grown to ~7.9k lines. Move it to
docs/changelog/ with one file per release, named with zero-padded
minor/patch versions (e.g. v1.11.00.md) so plain file browsers sort
them correctly. Add a README.md index (newest first) and a TEMPLATE.md
for cutting new releases. The root CHANGELOG.md now points to the new
location.
