Docs additions

[leburgel]

An attempt at improving consistency in spin and Pauli operator naming between the implementation and the documentation that got a little out of hand. Tried to:

• Systematically use Sⁱ for spin operators in model Hamiltonian docstrings. In particular, also introduced unicode aliases for these to be consistent with the model docstrings.
• Systematically use a⁺ and a⁻ for bosonic creation operators, both in the definitions in the documentation and in the model Hamiltonian docstrings.
• Systematically use e for spinful fermions.
• Explicitly add all unicode aliases to the docstrings.

I think all of the spinful fermion operators that take an up-down spin index also need an explicit docstring to be able to make sense of the Hubbard Hamiltonian, but I didn't try this (yet) since I'm not too sure about what's going on exactly.

[lkdvos]

Looks great, thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cbc20b9) 50.00% compared to head (3b40cde) 52.00%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #12      +/-   ##
==========================================
+ Coverage   50.00%   52.00%   +2.00%     
==========================================
  Files          16       16              
  Lines        1272     1275       +3     
==========================================
+ Hits          636      663      +27     
+ Misses        636      612      -24     

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