metaprobook new feature

Stream: lean4

[Asei Inoue]

Several of the PRs I submitted to the metaprogramming book have been merged, and the following new features have been added:
see: https://leanprover-community.github.io/lean4-metaprogramming-book/

• Table of contents within pages (not displayed on mobile devices)
• Jump from code blocks to Lean 4 Web
• Jump to Lean 4 Web from the button at the top-right of each page

I hope you enjoy these new features!