Fallback meta default help

[BrianPugh]

• If an App doesn't have explicit help text, it should fallback to app.default_command.__doc__, then app.meta.default_command, app.meta.meta.default_command, and so on.
• Also fixes displaying meta command help screen.
• Doc pages for "help" and "version".

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (93516ca) 92.39% compared to head (2b20ae0) 92.27%.

Files	Patch %	Lines
cyclopts/core.py	95.83%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
- Coverage   92.39%   92.27%   -0.12%     
==========================================
  Files          10       10              
  Lines        1052     1062      +10     
  Branches      248      252       +4     
==========================================
+ Hits          972      980       +8     
- Misses         45       46       +1     
- Partials       35       36       +1     

Flag	Coverage Δ
unittests	92.27% <95.83%> (-0.03%)	⬇️

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