[FLORA-448] Add description field in package index

[mau5mat]

Proposed changes

Contributor checklist

• [x] My PR is related to #448
• [x] I have read and understood the CONTRIBUTING guide
• [x] I have inserted my change and a link to this PR in the CHANGELOG

[mau5mat]

@mau5mat Looks like we forgot to change the tests: https://github.com/flora-pm/flora-server/actions/runs/7157839577/job/19489218962?pr=486#step:8:399

I can take a look in an hour or so

[tchoutri]

Thank you @mau5mat!!