ruff - check and fix escape-sequence-in-docstring

[pavoljuhas]

• ruff - activate D301 - escape-sequence-in-docstring

• Fix escape sequences in docstrings, e.g., switch to raw
  docstrings when we need literal backslash.

Partially implements #7505

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.36%. Comparing base (63cfe53) to head (5fe739a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7670      +/-   ##
==========================================
- Coverage   99.37%   99.36%   -0.01%     
==========================================
  Files        1082     1082              
  Lines       96691    96691              
==========================================
- Hits        96086    96081       -5     
- Misses        605      610       +5     

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[pavoljuhas]

This fixes funky effects of escape sequences in docstrings, for example, \r in
pydoc cirq.plot_density_matrix | cat:

# BEFORE

    2. The blue rectangles on the diagonal elements represent the probability
angle$.measuring the system in state $|i
    Rendering scheme is inspired from https://algassert.com/quirk

# AFTER

    2. The blue rectangles on the diagonal elements represent the probability
    of measuring the system in state $|i\rangle$.
    Rendering scheme is inspired from https://algassert.com/quirk

[mhucka]

Great!