Fix grammar and spelling issues in documentation and some outdated info

[tleonhardt]

Fix consistency, grammar, and spelling issues in documentation as well as some outdated info.

I made a detailed first-pass through the documentation focused on grammar, spelling, and consistency issues. I made a weak attempt to also correct any info which was very obviously outdated.

The documentation will definitely need another pass prior to the 3.0.0 release to make sure it adequately covers cmd2 in its current state. But this is an attempt to cleanup some of the low-hanging fruit first.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.81%. Comparing base (37b1b7d) to head (1de99e5).
⚠️ Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1509      +/-   ##
==========================================
+ Coverage   98.79%   98.81%   +0.02%     
==========================================
  Files          23       23              
  Lines        4961     4961              
==========================================
+ Hits         4901     4902       +1     
+ Misses         60       59       -1     

Flag	Coverage Δ
unittests	98.81% <ø> (+0.02%)	⬆️

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