docs(glossary) Create a Federated Learning glossary

[Julianrussmeyer]

Issue

Description

We want to open our Flower Glossary to contributions from our community.

Related issues/PRs

Proposal

Explanation

Checklist

• [x] Implement proposed change
• [ ] Write tests
• [ ] Update documentation
• [x] Make CI checks pass
• [ ] Ping maintainers on Slack (channel #contributions)

Any other comments?