Add how-to guide for using Flower Multi-run

[chongshenng]

Issue

Description

Our Flower Multi-run lack official feature documentation.

Related issues/PRs

N/A

Proposal

Explanation

Add a how-to guide highlighting the features of Flower Multi-run.

Checklist

• [X] Implement proposed change
• ~~[ ] Write tests~~
• [X] Update documentation
• [X] Update the changelog entry below
• [x] Make CI checks pass

Changelog entry

Any other comments?

N/A