Add link to community config tool

[zerebos]

This is something I talked with Mitchell about briefly around the time Ghostty first went public, but we wanted to add a link to the community configuration tool (tool: https://ghostty.zerebos.com) or (repo: https://github.com/zerebos/ghostty-config) to the docs since it's a tool the community finds very helpful. I know he wanted to wait until things settled down a bit before considering how best to integrate it and since it's been 3 months I figured I would put this on the queue to consider again.