📝 Add documentation for ZX-calculus package.

[pehamTom]

Description

This PR introduces documentation for the C++ ZX-calculus library.

Fixes #813

This change addresses concerns about missing documentation that came up in the revision openjournals/joss-reviews#7478 .

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.3%. Comparing base (68ea5fa) to head (c11c95d).

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #817   +/-   ##
=====================================
  Coverage   92.3%   92.3%           
=====================================
  Files        127     127           
  Lines      13509   13509           
  Branches    2080    2080           
=====================================
+ Hits       12476   12477    +1     
+ Misses      1033    1032    -1     

Flag	Coverage Δ
cpp	92.0% <100.0%> (+<0.1%)	⬆️
python	99.7% <ø> (ø)

Files with missing lines	Coverage Δ
include/mqt-core/ir/operations/Expression.hpp	92.7% <ø> (ø)
include/mqt-core/zx/Rational.hpp	96.2% <ø> (ø)
include/mqt-core/zx/Simplify.hpp	100.0% <ø> (ø)
include/mqt-core/zx/Utils.hpp	100.0% <ø> (ø)
include/mqt-core/zx/ZXDefinitions.hpp	92.0% <ø> (ø)
include/mqt-core/zx/ZXDiagram.hpp	95.7% <ø> (ø)
src/zx/Simplify.cpp	97.1% <100.0%> (ø)
src/zx/ZXDiagram.cpp	91.9% <ø> (ø)

... and 1 file with indirect coverage changes

[burgholzer]

These look phenomenal ✨ many thanks for taking the time to add these docstrings!
I just browsed through them and added a couple of inline comments where I noticed that something might be slightly off. Hope these help.

Looking forward to seeing this in the online documentation!