Doc/Michelson: redirect operations on unions to interactive reference

Context

Answers this part of #4077:

• delete the parts about instructions from the static reference:
  • Operations on unions

The documentation about all the operations on union in the interactive reference contains that found in the static reference.
The inclusion was checked manually by comparing the static and interactive documentations for each type and comparison instruction of the static reference.

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

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