Started filling in some missing stuff in the new documentation structure

[tleonhardt]

Changes include:

• Removed a couple TODO-style lines from the index which looked like they had been done
• Added info to the "why migrate to cmd2" page
• Added missing info to the "incompatibilities" page

[codecov]

Codecov Report

Merging #715 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #715   +/-   ##
=======================================
  Coverage   96.03%   96.03%           
=======================================
  Files          12       12           
  Lines        3356     3356           
=======================================
  Hits         3223     3223           
  Misses        133      133

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