CHORE: update CHANGELOG for v0.18.1

[pyansys-ci-bot]

Update CHANGELOG for v0.18.1 and remove .md files in doc/changelog.d.

Caution

Do not merge this pull request until the release has been successfully made.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.31%. Comparing base (d27bbdd) to head (355923e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6503   +/-   ##
=======================================
  Coverage   82.31%   82.31%           
=======================================
  Files         231      231           
  Lines       74455    74455           
=======================================
+ Hits        61286    61287    +1     
+ Misses      13169    13168    -1     

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