PyPI readme: fix screenshot links & trim changelog

[hynek]

I was super honored to learn that my favorite http library is using my hatch-fancy-pypi-readme!

When adding it to my brag list, I've noticed two problems in your usage and wanted to offer you the simple solutions:

1. Your relative image URLs from the README 404 – this is why I've added the substitutions feature (if the logo is an absolute link just for PyPI, I can also make it relative if you'd like).
2. Your Changelog is really long so I would recommend trimming it. I've added the technique that I use personally.

You can test the result by running pipx run hatch-fancy-pypi-readme, but here's how the tail looks like after this PR:

...

A huge amount of credit is due to `requests` for the API layout that
much of this work follows, as well as to `urllib3` for plenty of design
inspiration around the lower-level networking details.

---

<p align="center"><i>HTTPX is <a href="https://github.com/encode/httpx/blob/master/LICENSE.md">BSD licensed</a> code.<br/>Designed & crafted with care.</i><br/>&mdash; 🦋 &mdash;</p>

## Release Information

### Added

* Support for Python 3.11. (#2420)
* Allow setting an explicit multipart boundary in `Content-Type` header. (#2278)
* Allow `tuple` or `list` for multipart values, not just `list`. (#2355)
* Allow `str` content for multipart upload files. (#2400)
* Support connection upgrades. See https://www.encode.io/httpcore/extensions/#upgrade-requests

### Fixed

* Don't drop empty query parameters. (#2354)

### Removed

* Upload files *must* always be opened in binary mode. (#2400)
* Drop `.read`/`.aread` from `SyncByteStream`/`AsyncByteStream`. (#2407)
* Drop `RawURL`. (#2241)


---

[Full changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)

I think that's better, no? I you don't like it, I can remove it again and just contribute the image fix.

P.S. if there's anything else I can help you with the PyPI readme, pls lmk.

[tomchristie]

Lovely stuff, thanks @hynek. 💚

I should've got this in for our 0.23.2 release just now but never mind - let's get it merged now, and check the PyPI info on our next release.