docs: add a migration guide for 0.9.0, and drop merge noise from the changelog

Twenty-one breaking changes in one release, with a live consumer. The
changelog lists them; `MIGRATING.md` says what to do about them, leading
with the three that change what an existing, *compiling* call returns —
unknown keys, predictions from a broken fit, and `Gaussian`'s operators
— since those are the ones the compiler will not find for you.

Every "after" snippet was compiled, not written from memory, and doing
so caught three errors in my own guide:

- `log_evidence_for(&[&"alice"])` does not compile at `K = String`. The
  right spelling is `&["alice"]`, which works at *both* key types —
  checked, because a guide that is right for half its readers is worse
  than no guide.
- the same for `filtered_log_evidence_for`
- the `Analysis<'h> { joint: Joint<'h> }` example needs a history at the
  default key type; pairing it with a `History<String>` does not compile

git-cliff skips merge commits now. Every branch lands with `--no-ff`, so
a release's merges outnumber its real commits and say nothing the merged
ones do not — 0.9.0's changelog had fourteen lines of them under "Other
(unconventional)". `ci:` commits get a group instead of falling through
to that catch-all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011hcFjNDmHXZF8URGLku5zZ

README.md

@@ -419,7 +419,8 @@ expensive than `quality()`. Scoring every pairing among `n` competitors is
 Every box on the old todo list is ticked, so it has been retired; open work
 lives in the issue tracker instead. The crate is in use and the API is still
 moving — breaking changes are batched into minor releases rather than dribbled
-out, and `CHANGELOG.md` records them.
+out. `CHANGELOG.md` lists them and [`MIGRATING.md`](MIGRATING.md) explains what
+to do about them.
 
 ## License