Update the circuit library docs

[Cryoris]

e.g. why we use different gate types and what's the advantages

Summary

With #13046 the designated workflow with the circuit library changed. This PR adds a section to the docs explaining this new structure. While not fully done, I'm already opening this draft PR for people to have a look at the text.

Details and comments

This includes some other minor changes:

• add a TOC for the different sections
• consistently sort the standard gates (before this was partially sorted, partially chaotic)
• move the documentation of get_standard_gate_name_mapping under Standard gates
• always mention the circuit functions and gates before the old circuits

To do

• Add some more examples to the sections
• Move the template circuits to a separate page
• Can we find a better name/structure for "particular quantum circuits"?

[coveralls]

Pull Request Test Coverage Report for Build 12137580520

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 16 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.008%) to 88.935%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	4	92.23%
crates/qasm2/src/parse.rs	12	97.15%

Totals
Change from base Build 12131777933:	-0.008%
Covered Lines:	79378
Relevant Lines:	89254

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💛 - Coveralls

[ShellyGarion]

Currently all the standard gates are ordered alphabetically. Does it make sense to order them according to the number of qubits (0,1,2,3,many) and then maybe according to the number of parameters (0,1,2,3)?

[ShellyGarion]

copy here a comment about organizing the oracles (grover, phase, bitflip) in their own section:

#13769 (comment)

[mtreinish]

Since this is just documentation it can merge after 2.0.0rc1 is tagged before the final release. I've pre-emptively tagged it as stable backport potential because we'll need to backport it to stable/2.0 once that's created when we tag 2.0.0rc1.