Rework QPY docs

[ElePT]

Summary

Attempts to fix/improve #9967.

Details and comments

The QPY doc used to incrementally "build up" with every QPY release in a way that made it hard to get an overview of what the format looks like now. In this PR, I try to improve the readability fixing this issue and adding a few more changes, namely:

1. "squashed" all versions from the previous doc into a single overview, while still providing some context of when were certain elements added to the format
2. added a "version history" table that allows people to see in which terra release was QPY bumped to which version. This is to compensate for the loss of historical tracking in the doc, people can always inspect the releasenotes or release artifacts and see what the code looked like then (note, I have double checked a couple of times, and still concluded that QPY v.3 and v.4 were both part of the same release. Correct me if I'm wrong)
3. added more headers, structure to the text, and where possible tried to add a schematic overview of what each block looks like (HEADER | DATA | ...)
4. reviewed grammar, spelling, tried to unify notation

Note: The local build/CI does not show document headings after a certain level ( the ones with ------- or ==== are not shown) in the sidebar menu, but the current deployed docs do show them, at least until (----). I am assuming there is some difference in the Sphinx theme that doesn't show them.

[coveralls]

Pull Request Test Coverage Report for Build 5336201879

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 229 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-0.03%) to 85.93%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/initializer.py	1	97.3%
crates/qasm2/src/lex.rs	3	90.63%
qiskit/circuit/library/data_preparation/state_preparation.py	7	96.35%
crates/qasm2/src/parse.rs	12	96.65%
qiskit/dagcircuit/dagcircuit.py	21	89.64%
qiskit/visualization/circuit/matplotlib.py	185	57.41%

Totals
Change from base Build 5313232499:	-0.03%
Covered Lines:	71276
Relevant Lines:	82947

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💛 - Coveralls

[mtreinish]

I've only done a quick high level skim over this so far. At a high level this reorganization is a huge improvement over the structure we had in the format docs before. Thanks for doing this!

What was missing before was the top overview of the current format. The historical artifacts are important for anyone potentially writing a deserializer for backwards compatibility. But for people trying to figure out how the format works and what terra is emitting today we had the order of everything backwards.

The one thing I also wanted to call out is that this will have a conflict with #10148 which is adding version 8. I wrote the docs there in the existing format. But we can handle the conflict based on what merges first I guess.

[mtreinish]

Maybe link to the section too here. For circuit it's easy because it's right below. But for schedule block it's a lot of scrolling :)

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @mtreinish
• @nkanazawa1989