Improve Quickstart: Circuits

[mariaschuld]

Updates the circuits section of the quickstarts, as part of a minor overhaul.

Bigger changes:

• Merge sections on wires and shots
• Add reference to draw_mlp
• Delete QNodeCollections section

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2534 (e972b3e) into master (fb797be) will increase coverage by 3.74%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2534      +/-   ##
==========================================
+ Coverage   95.78%   99.53%   +3.74%     
==========================================
  Files         243      243              
  Lines       19405    19405              
==========================================
+ Hits        18588    19314     +726     
+ Misses        817       91     -726     

Impacted Files	Coverage Δ
pennylane/_qubit_device.py	98.73% <0.00%> (+0.31%)	⬆️
pennylane/devices/default_qubit.py	100.00% <0.00%> (+0.38%)	⬆️
pennylane/utils.py	97.97% <0.00%> (+0.67%)	⬆️
...ane/transforms/optimization/single_qubit_fusion.py	100.00% <0.00%> (+1.92%)	⬆️
pennylane/templates/embeddings/amplitude.py	98.00% <0.00%> (+2.00%)	⬆️
pennylane/gradients/vjp.py	100.00% <0.00%> (+2.89%)	⬆️
pennylane/interfaces/torch.py	100.00% <0.00%> (+3.27%)	⬆️
pennylane/ops/qubit/qchem_ops.py	100.00% <0.00%> (+4.81%)	⬆️
pennylane/devices/default_qubit_torch.py	92.07% <0.00%> (+5.94%)	⬆️
pennylane/ops/qubit/parametric_ops.py	100.00% <0.00%> (+6.68%)	⬆️
... and 19 more

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[josh146]

Left some comments and suggestions, but nothing blocking! (except probably the mention of lightning)