Align dir structure and URL structure with left nav hierarchy

[ppiegaze]

Why are the changes needed?

We want to align the directory structure in flyte/docs and the URL structure with the hierarchy of the left nav.

This will make adding and re-organizing content more straightforward

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4223a83) 58.98% compared to head (843bf79) 59.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4843      +/-   ##
==========================================
+ Coverage   58.98%   59.03%   +0.04%     
==========================================
  Files         644      644              
  Lines       55146    54786     -360     
==========================================
- Hits        32529    32343     -186     
+ Misses      20043    19869     -174     
  Partials     2574     2574              

Flag	Coverage Δ
unittests	59.07% <ø> (+0.08%)	⬆️

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