docs: rework roadmap

[CyanVoxel]

Summary

This PR includes a complete rework of the "Roadmap" page to achieve the following:

• Focus on feature groups rather than version milestones
  • Version milestones frequently shift around, and as time has gone on most of the feature distribution has shifted towards shipped or near future versions
  • The GitHub Milestones page is a much more accurate view of upcoming features
  • This significantly cleans up the table of contents and makes these features easier to find overall
  • Version estimates are still present, but now a part of the individual features themselves. Shipped features have the estimates converted to changelog links.
• Redesign the priority labels to become small color-coded icons with tooltips
• Add additional feature steps and remove outdated or unnecessary ones
• Rename from "Feature Roadmap" to simply "Roadmap"
  • Not all items were necessarily features, and the name could be simplified

Tasks Completed

• Platforms Tested:
  • Windows x86
  • Windows ARM
  • macOS x86
  • macOS ARM
  • Linux x86
  • Linux ARM
• Tested For:
  • Basic functionality
  • PyInstaller executable

[Computerdores]

I didn't notice any issues while reading it over, but reviewing text isn't my strong suit