Fix CI: update Typst to 0.13.0 for fletcher compatibility

The fletcher:0.5.8 package requires Typst 0.13.0 or newer.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: GiggleLiu

.github/workflows/docs.yml

@@ -30,7 +30,7 @@ jobs:
 
       - name: Install Typst
         run: |
-          curl -sSL https://github.com/typst/typst/releases/download/v0.12.0/typst-x86_64-unknown-linux-musl.tar.xz | tar -xJ
+          curl -sSL https://github.com/typst/typst/releases/download/v0.13.0/typst-x86_64-unknown-linux-musl.tar.xz | tar -xJ
           mv typst-x86_64-unknown-linux-musl/typst "$HOME/bin/"
 
       - name: Build mdBook