Docs: replace bitly links

[merobi-hub]

Problem

The Slack links use bitly, which we don't need as permanent links are available. The bitly subscription is expiring soon.

Solution

This replaces the bitly invite links with permanent ones.

Note: All database schema changes require discussion. Please link the issue for context.

One-line summary: replace bitly links with permanent invites

Checklist

• [x] You've signed-off your work
• [ ] Your changes are accompanied by tests (if relevant)
• [x] Your change contains a small diff and is self-contained
• [ ] You've updated any relevant documentation (if relevant)
• [x] You've included a one-line summary of your change for the CHANGELOG.md (Depending on the change, this may not be necessary).
• [ ] You've versioned your .sql database schema migration according to Flyway's naming convention (if relevant)
• [ ] You've included a header in any source code files (if relevant)

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