chore: major docs update, image migration, and site modernization

[Stensel8]

Summary

• Migrate Hextra theme from git submodule to Go module (go.mod/go.sum); remove .gitmodules entry
• Update Hugo and Hextra dependencies
• Replace supergfxctl with asusctl armoury throughout; supergfxctl is deprecated; GPU switching now handled via asusctl armoury set/get dgpu_disable
• Add "Why CachyOS?" section and CachyOS Kernel Manager (scx_lavd / sched_ext) to getting-started
• Add scx_lavd (sched_ext) CPU scheduler to system config table on homepage
• Remove news page; content integrated into relevant pages
• Expand known-issues: active issues, GPU/display bugs (PSR, amdgpu page faults), NVIDIA soft lockup, Brave crashes, YubiKey/FIDO2, GDM autologin, and VM troubleshooting
• Add Smile emoji picker screenshots and documentation to applications
• Add AVIF (quality 80, speed 6); document batch conversion in README
• Update domain from zephyrus-linux.stentijhuis.nl to zephyrus-linux.stensel.nl in all files
• Add contributor welcome message to CONTRIBUTING.md

Type of change

• feat — new page or feature
• fix — bug fix (broken link, incorrect command, layout issue)
• content — update or improve existing content
• docs — changes to CONTRIBUTING, README, or meta documentation
• chore — maintenance (dependencies, config, CI/CD)
• refactor — restructuring without content changes
• style — formatting, whitespace, typos

Checklist

• PR title follows the commit convention (e.g. fix: correct nmcli command in eduroam guide)
• Both EN and NL versions updated (if applicable)
• Images are in AVIF format (not PNG/JPG)
• No broken image references (/images/*.avif all exist in static/images/)
• Tested locally with hugo server