Doc/Michelson: link in macros introduction

Manually testing the MR

make -C docs -j

And then open docs/_build/alpha/michelson.html in a web browser.

Or open the exposed artifacts Documentation - excluding old protocols below, and navigate to docs/_build/alpha/michelson.html.

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR