Official nixpkgs app badge

[pbsds]

Currently flathub and appimages excel in distributing software to non-technical users. Users have been conditioned to look for badges rather than download links, and it would be great if nixpkgs had an official one too.

Examples of badges from various stores/repositories:

• https://flathub.org/en/badges
• https://f-droid.org/docs/Badges/
• https://developer.apple.com/app-store/marketing/guidelines/
• https://partnermarketinghub.withgoogle.com/brands/google-play/visual-identity/badge-guidelines/
• https://apps.microsoft.com/badge?hl=en-US&gl=NO

Example of how a project may use such badges:

• https://akademia.geteduroam.no/app/
• https://mattermost.com/apps/

(For a good user experience with badges is we also miss a per-package page for badges to link to, but a search.nixpkgs.org url may suffice for now)