Add note about Gamehub compatibility - resolves issue #738

[Ampersandstorm]

• [ ] Intro paragraph for Steam header
• [ ] Cleaned up and clarified titles
• [ ] Proton section - link to proton official site or documentation
• [ ] Capitalization for Proton, Flatpak, and GameHub
• [ ] Added note and link to check game-specific user feedback using Protondb
• [ ] Added note regrading pop os min verison for gamehub support
• [ ] Reorg content in the protonup intro section for clarity
• [ ] Added ProtonUp-Qt installation instructions to keep content consistant
• [ ] Added headers to protonup section to clarify sections

Should we include a section about using Bottles?

[storyaddict]

Bottles might be a good addition. It still seems a little rough around the edges though. Really cool project, but maybe we do another PR soon to add it instead of this one.