feat: switch to pre-built binaries instead of compiling from source

[MichaelFisher1997]

Summary

This PR changes the Flatpak build process to download and package pre-built Redot Engine binaries instead of compiling from source, resulting in
significantly faster builds and reduced dependencies.

Key Changes

🚀 Binary Distribution Approach

• Updated manifest (org.redotengine.Redot.yaml:112-116) to download official pre-built Linux binaries from GitHub releases
• Removed source compilation - no longer builds Redot from source code
• Faster builds - download and package instead of compile (saves ~30+ minutes)
• Reduced build dependencies - only needs basic packaging tools, not full compilation toolchain

🤖 Automated Release Management

• New GitHub Action (.github/workflows/update.yaml) to automatically detect new Redot Engine releases
• Auto-updates manifest with new binary URLs and SHA256 hashes when releases are published
• Creates PRs automatically for maintainer review and merging

📚 Updated Documentation

• Revised README to reflect the new binary-only approach
• Fixed installation instructions with proper Flathub remote setup and prerequisites
• Clarified build process - emphasizes fast binary packaging vs. slow source compilation

Before vs After

Before (Source Build):

• Downloads Redot source code
• Compiles with SCons build system
• Requires full development dependencies
• Build time: 30+ minutes

After (Binary Package):

• Downloads official pre-built binary (Redot_v4.3.1-stable_linux.x86_64.zip)
• Extracts and packages with runtime dependencies
• Minimal build requirements
• Build time: 2-3 minutes

Testing Needed

Please test the updated build process:

1. Clean Build Test:

   git clone --recursive https://github.com/Redot-Engine/org.redotengine.Redot.git
   cd org.redotengine.Redot/
   flatpak remote-add --if-not-exists --user flathub https://dl.flathub.org/repo/flathub.flatpakrepo
   flatpak install --user flathub org.freedesktop.Sdk//24.08 -y
   flatpak-builder --user --install --force-clean build-dir org.redotengine.Redot.yaml
   

2. Verify Functionality:
   flatpak run org.redotengine.Redot
3. Confirm Build Speed:
   - Should complete in 2-3 minutes vs. previous 30+ minute source builds
   - Verify the binary is properly packaged and launches correctly

Benefits

• ⚡ Dramatically faster builds (2-3 min vs 30+ min)
• 🔧 Reduced build complexity - no compilation dependencies
• 🤖 Automated maintenance - auto-updates when new releases are published
• ✅ Same end result - identical Redot functionality with official binaries
• 🏗 More reliable builds - uses tested release binaries instead of source compilation