chore: reorganize docs (update sql-> func)

[yathomasi]

• Relevant link: slack discussion
• Preview URL: https://reorg-doc.datachain-documentation.pages.dev

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.64%. Comparing base (db97a68) to head (33f437d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #844   +/-   ##
=======================================
  Coverage   87.64%   87.64%           
=======================================
  Files         128      128           
  Lines       11448    11448           
  Branches     1542     1542           
=======================================
  Hits        10034    10034           
  Misses       1026     1026           
  Partials      388      388           

Flag	Coverage Δ
datachain	87.56% <ø> (ø)

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[yathomasi]

@skshetry @shcheklein PTAL