Split the changelog into separate files (#18725)
Summary
--
During the release today, I noticed that the changelog is finally too
long to
render at all on GitHub. This PR follows the same splitting procedure as
in
uv (astral-sh/uv#11510, astral-sh/uv#12099): first splitting the file
into one
per minor version, and then reversing the contents of each file to start
with
the breaking release (`changelogs/0.11.x.md` starts with 0.11.0 instead
of
0.11.13 as in the old changelog).
For the second part, I used
[`reverse-changelog.py`](https://github.com/astral-sh/uv/blob/main/scripts/reverse-changelog.py)
from the uv repo, so hopefully everything is correct. I spot-checked
0.7.0 at least.
