Fix placeholder rst hook bugs

[dylanhmorris]

Closes #364 but does not resolve this, which will require a larger rethink of the hook script:

It might also be nice to update the hook so that it fails if the.rst files exist but are not identical to the expected placeholders.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.24%. Comparing base (ba30553) to head (9127df3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #365   +/-   ##
=======================================
  Coverage   93.24%   93.24%           
=======================================
  Files          39       39           
  Lines         918      918           
=======================================
  Hits          856      856           
  Misses         62       62           

Flag	Coverage Δ
unittests	93.24% <ø> (ø)

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[gvegayon]

Mostly one comment about counting changes when RSTs are removed. Before closing/merging this, I would also create a separate issue for improving this whole process (compare expected vs observed RST).

[gvegayon]

LGTM