please update docs/packaging.md to cover Debian packaging

Hi,

Please consider updating `docs/packaging.md` to include something like this:
```markdown
**Debian**
- package info: https://packages.debian.org/emscripten
- package development info: https://tracker.debian.org/emscripten
- issue tracking: https://bugs.debian.org/emscripten
```
If you insist on putting a name there, then I am `Jonas Smedegaard ` but really that might change over time and promoting abstract Debian references rather than persons works more reliably in my experience.

Thanks to @sbc100 for encouraging me to add that info myself in #12381
I choose to only propose this change not provide an MR because I refrain from ever doing a `git push` to Github due to their [anti-copyleft terms of service](https://joeyh.name/blog/entry/removing_everything_from_github/).
I am happy to provide a git clone elsewhere for you to cherry-pick from - please do tell if that would be helpful to you.