Made changes to improve getting started tutorial

[Guillermo-Mijares-Vilarino]

Summary

Made some changes to the Getting Started with Qiskit 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.
• Added a link to an explanation of what Greenberger-Horne-Zeilinger (GHZ) states are. In the tutorial they are mentioned as if they were known by users but it may not be the case.
• Changed the titles of some steps so they are actions instead of nouns.
• Explained that when using QuantumCircuit.draw('mpl') we are using Matplotlib and showed the difference between using and not using Matplotlib. Originally the 'mpl' option appeared without any acknowledgement.
• Mentioned the possibility of reversing the order of the qubits when drawing a circuit so the most significant qubits are at the top and showed it with a quick example. Even if it goes slightly against the checklist (better to not provide options) I believe it can make a big difference for people who have more Physics-oriented backgrounds (like me) and are used to drawing circuits that way. I really appreciated that option when I got started with Qiskit.
• Expanded a bit the explanation on building the circuit with the measures. In particular, I explained what the barrier does
• Corrected a typo in which it is said that get_counts(circuit) is a function instead of a method.
• Put some attention to the fact that we are transpiling the circuit before running it.
• Added a brief conclusion about what the user has achieved with this tutorial.
• Changed the first sentence so it's more focused on the user. Our guidelines state that it's preferable to involve the user by saying "you are going to do x" instead of "we are showing you x"
• Removed references to statevector_simulator, qasm_simulator and unitary_simulator and focused on only one example of AerSimulator.
• Added links to API references for gates, simulators and visualization options.
• Removed references to deprecated execute function and changed them to references to run method.
• Added a "Visualize the results" section.

Fixes Qiskit/qiskit#1520

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[HuangJunye]

@Guillermo-Mijares-Vilarino I left some comments. I haven't read through the whole tutorial yet. I will finish the review next week.

[HuangJunye]

This is staled. Closing it.