docs(framework:skip) Add FLWR_VERSION_REF to docs

[Robert-Steiner]

Issue

Description

Adds FLWR_VERSION_REF (https://github.com/adap/flower/pull/4144) to the contributor docs

Related issues/PRs

Proposal

Explanation

Checklist

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

Any other comments?