Transformation docstrings fixes

[Yoshanuikabundi]

This fixes the warnings causing RTD to choke on OpenFreeEnergy/openfe#493 and also adds Transformation and NonTransformation to the GUFE API reference (for good measure).

Previously, several properties in both classes had docstrings AND an Attributes entry in the class docstring. This meant that they were documented twice, which Sphinx gets grumpy about. The documentation was also not identical, so readers noticing one might have missed information present only in the other. I've consolidated the documentation and type hints into the property docstrings and annotations. I've also fixed some errors arising because ReST is very picky about trailing ss after code spans and italicised regions and so on - so for example, I've changed `ChemicalSystem`s to `ChemicalSystem` objects

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (8d9fb2e) 99.09% compared to head (9413007) 99.09%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #210   +/-   ##
=======================================
  Coverage   99.09%   99.09%           
=======================================
  Files          36       36           
  Lines        1771     1771           
=======================================
  Hits         1755     1755           
  Misses         16       16           

Impacted Files	Coverage Δ
gufe/transformations/transformation.py	100.00% <100.00%> (ø)

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