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%

---

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

---

💛 - Coveralls