Add code example in Operator docstring (backport #12985)

[mergify]

Summary

Adds code example in the docstring of the Operator class.

Details and comments

Adds the following code example which applies an $X$ operator to the zero state:

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This is an automatic backport of pull request #12985 done by [Mergify](https://mergify.com).

[qiskit-bot]

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[coveralls]

Pull Request Test Coverage Report for Build 10468442951

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.826%

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Totals
Change from base Build 10468372678:	0.02%
Covered Lines:	66806
Relevant Lines:	74373

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💛 - Coveralls