Made changes to improve operators tutorial

[Guillermo-Mijares-Vilarino]

Summary

Made some changes to the Summary of Quantum Operations tutorial to make it follow more closely our documentation checklist and guidelines.

Details and comments

• Moved imports to where they are strictly needed so the users can better understand what imports are needed for each step.
• Corrected the controlled-$u$ matrix form.
• Added matrices for most significant bit controlled Hadamard, Rz, U, Toffoli and Fredkin gates for more Physics-oriented users and consistency with most sources.
• Explained what phase and Pauli gates meant in terms of rotation on the Bloch sphere.
• Mentioned where each simulator will be used in the introduction.
• Slightly changed the introduction so it's more focused on the user (using "you will do x" instead of "we will show you x")
• Added comments to code.
• Added a brief conclusion about what the user has achieved with this tutorial.
• Run the notebook.

Fixes Qiskit/qiskit#1522

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[HuangJunye]

This is superseded by

1. https://github.com/Qiskit/qiskit-terra/pull/8370
2. https://github.com/Qiskit/qiskit-terra/pull/8391
3. https://github.com/Qiskit/qiskit-terra/pull/8443