chore: formalize book.js patching

[biplavbarua]

Formalize book.js Patching

Description

This PR formalizes the process of patching book.js.

Previously, theme/book.js contained custom modifications (Google Analytics) but there was no clear separation from the upstream mdBook code, and third_party/mdbook/book.js was a symlink to this modified file.

This change:

1. Restores third_party/mdbook/book.js to its pristine upstream state (matching mdBook version used).
2. Adds a patches/book.js.patch file containing the custom Google Analytics integration.
3. Adds a new xtask patch-assets command to automatically apply this patch.
4. Updates xtask install-tools to run patch-assets, ensuring the environment is always correctly set up.

Verification

• Run cargo xtask patch-assets to verify the patch applies cleanly.
• Inspect theme/book.js to confirm gtag calls are present.
• Run cargo xtask serve to verify the book builds and serves correctly.

Fixes #2924