docs: redirect GitHub Pages docs to GitBook

[angpt]

Description

Adds redirects to Gitbook from old GitHub pages docs

PR Type

• 📚 Documentation
• 🚦 Infrastructure

Checklist

• I understand the code I am submitting.
• I have run this code locally and verified the change.
• New and existing tests pass locally, or I have explained why tests were not run.
• Documentation was updated where necessary.
• If I changed code in llama.cpp/, whisper.cpp/, or stable-diffusion.cpp/, I also updated the matching *.patches/ files.
• I have read and followed the contribution guidelines.
• AI Usage:
  • No AI was used.
  • AI was used in an assistive capacity.
  • This PR includes substantial AI-generated content.

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[aittalam]

TY @angpt ! 🙏