metapro book, #136, "run on Lean4 playground"

Stream: PR reviews

[Asei Inoue]

@Julian Berman Please review this PR:
https://github.com/leanprover-community/lean4-metaprogramming-book/pull/136

[Asei Inoue]

PR summary:

This PR makes the mdbook interactive.

• add "run on Lean4 playground" button to each lean code block
• add "run on Lean4 playground" button to each page

Objective:

• Make it easy to run code examples