docs: add "Release highlights" section to latest release (v0.24.0)

As per a [Discord discussion](https://discord.com/channels/968932220549103686/968932220549103689/1314291772893171784), we thought it might be nice to include additional information in the changelog/release notes, similar to certain other open-source projects.

For example: In [the Rust 1.82 release notes](https://blog.rust-lang.org/2024/10/17/Rust-1.82.0.html), they include a one-line description of the project as well as installation instructions, and then go over several release highlights.

Possible future process:

- We can put the release highlights into `CHANGELOG.md` itself, so that it can undergo the normal review process.
- We'll add the static description/installation text to each release as we publish it (which doesn't need to be duplicated for each version in `CHANGELOG.md`).

CHANGELOG.md

@@ -8,6 +8,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Release highlights
+
 ### Breaking changes
 
 ### Deprecations
@@ -18,6 +20,16 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [0.24.0] - 2024-12-04
 
+### Release highlights
+
+* New [`jj absorb`](https://martinvonz.github.io/jj/latest/cli-reference/#jj-absorb) command automatically squashes changes from the current commit into relevant ancestor commits.
+
+* Experimental dynamic shell completions have been added; see [the docs](https://martinvonz.github.io/jj/latest/install-and-setup/#command-line-completion) for configuration.
+
+* [`jj duplicate`](https://martinvonz.github.io/jj/latest/cli-reference/#jj-duplicate) now accepts `--destination`/`--insert-before`/`--insert-after`.
+
+* Some deprecated commands have been removed (`jj move`, `jj checkout`, `jj merge`).
+
 ### Breaking changes
 
 * `jj move` has been removed. It was deprecated in 0.16.0.