docs: add imports in some examples

[hadrien]

Problem

• Examples in doc without imports

Solution

• Add imports

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3b93a98) to head (4b26774).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #22   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          100       100           
  Branches         6         6           
=========================================
  Hits           100       100           

Flag	Coverage Δ
default	100.00% <ø> (ø)
sqlmodel	98.00% <ø> (ø)

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