[DOCS] Document and add tests for not (!) operator

[simonschaufi]

No description provided.

[s2b]

Thank you for this contribution.

I would move the example about the not operator after the paragraph about comparisons. Other than that this looks good to me.

[simonschaufi]

The new example code only follows line 18+19: As a condition is a boolean value, you can just use a boolean argument. Alternatively, you can write a boolean expression there. Otherwise the whole structure needs to be reorganized.