Use more intuitive icons on roadmap

Upon initially seeing the roadmap, I misinterpreted the "x". I think some unicode emoji would help fix that.

Instead of "x" you could use: ✅
Instead of "*" you could use: 🚧
Instead of "i" you could use: ❌

It seems that the older roadmap did something similar. https://dioxuslabs.com/learn/0.3/roadmap.html

If the newer roadmap is preferred, maybe just put the legend above the table?

Happy to make the PR myself, if requested.

Compare the current roadmap with the older roadmap at https://dioxuslabs.com/learn/0.3/roadmap.html, then locate the newer roadmap source in the docsite. Update the roadmap symbols or add a legend above the table so the meanings of completed, in-progress, and unavailable items are immediately clear.

Written by the indexing model from the issue text.