Add a tip for how to use Yazi in Helix directly without Zellij or tmux

Following https://github.com/sxyazi/yazi/pull/2461 and [Yazi v25.4.8](https://github.com/sxyazi/yazi/releases/tag/v25.4.8), we can now launch Yazi directly from Helix.

This should be mentioned in the documentation.