Fold the Bundler changelog into CHANGELOG.md

Move the 4.0.0 and later sections of CHANGELOG-bundler.md under the
`### Bundler` heading of the matching CHANGELOG.md section, leaving
every entry as it was published apart from the bullet marker the 4.0.0
to 4.0.8 sections used. The pre-4.0 Bundler history, which used its own
version numbers, moves verbatim to doc/CHANGELOG-bundler-2.x.md, and the
cross-links between the two files are absolute so they still resolve
from inside the gem and from rdoc, neither of which ships doc/.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hsbt