📝 Rework existing project documentation

[burgholzer]

Description

This PR reworks most of the existing project documentation. Some of this was triggered by the ongoing JOSS submission, some was long overdue.
While extending the documentation, a small bug in the OpenQASM dump of classic controlled operations has been found and subsequently fixed.

While this PR mostly touches existing documentation, a follow-up PR will add documentation for the DD and the ZX package.

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 92.2%. Comparing base (653c606) to head (5ca5391).
Report is 8 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #789   +/-   ##
=====================================
  Coverage   92.2%   92.2%           
=====================================
  Files        125     125           
  Lines      13759   13764    +5     
  Branches    2165    2167    +2     
=====================================
+ Hits       12696   12701    +5     
  Misses      1063    1063           

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

Files with missing lines	Coverage Δ
...-core/ir/operations/ClassicControlledOperation.hpp	96.9% <100.0%> (+0.2%)	⬆️