Doc/Michelson: add a section on views

Context

In the Michelson documentation, a part of the views description that is being migrated to the interactive reference contains generic considerations that could better be extracted to a dedicated section of the language reference.

This part is in commit michelson-reference!123 (b10965a7), up to and including the paragraph about forbidden types.

We add a dedicated page for these explanations. It's a copy/paste of the contents explained above, with this fix-up, and some section titles added.

Manually testing the MR

make -C docs -j

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

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

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