Discord invite links are dead across the org

The Discord invite in the README of `carp_mobile_sensing`, `carp_core` and 13 other packages here is expired:

```
$ curl "https://discord.com/api/v10/invites/NKuUwCsV?with_counts=true"
{"message": "Unknown Invite", "code": 10006}
```

Those READMEs are what pub.dev renders, so it's on the public package pages too.

Two other codes are dead the same way, `PyxCmqS8` in `carp-docs-mintlify/docs/docs.json` and `Jzjd7aYg` in the carp.dk footer. Looks like they were time-limited rather than permanent invites. A permanent one would fix all three, but I don't know whether the server still exists - @bardram do you know?

Inventory the README files for `carp_mobile_sensing`, `carp_core`, and the other packages, plus `carp-docs-mintlify/docs/docs.json` and the carp.dk footer. First confirm with @bardram whether the Discord server still exists and obtain a permanent invite; replace the three expired codes and verify the links no longer return Discord's `Unknown Invite` response.

Written by the indexing model from the issue text.