Add changelog for the unreleased 0.15 (rerun-io#5809)

### What
Slightly unconventionally I decided to add the changelog under the `#
Unreleased` heading so we can merge this right away, without having to
wait for a release.


[Rendered](https://github.com/rerun-io/rerun/blob/emilk/0.15.0-changelog/CHANGELOG.md)

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5809)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5809?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5809?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5809)
- [Docs
preview](https://rerun.io/preview/8aed4a43cc19d8f59ea4cad14884c223f14a96fd/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/8aed4a43cc19d8f59ea4cad14884c223f14a96fd/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

---------

Co-authored-by: Clement Rey <[email protected]>
Co-authored-by: Antoine Beyeler <[email protected]>
Co-authored-by: Antoine Beyeler <[email protected]>
Co-authored-by: Jeremy Leibs <[email protected]>