Update readme

[tleonhardt]

For a long time I've felt like our top-level README needed a major overhaul.

I felt like it was light on useful information and delayed some of the most useful info until far down the page.

This moves the quickstart example much higher up, has it use the annoated decorator, adds an animated gif of using that toy application and removes the other images that were there.

It also attempts to put our best foot forward in the Why cmd2? section and does a better job of highlighting key features to aid with discoverability for new users.

Also:

• Updated the getting_started.md documentation page to more closely match the current state of the getting_started.py example and added a simple animated gif.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.66%. Comparing base (f363707) to head (5d50825).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1739   +/-   ##
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  Coverage   99.66%   99.66%           
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  Files          23       23           
  Lines        5960     5960           
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  Hits         5940     5940           
  Misses         20       20           

Flag	Coverage Δ
unittests	99.66% <ø> (ø)

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