Add datamodel-code-generator link in pydantic document site

[koxudaxi]

Change Summary

This PR datamodel-code-generator link in pydantic document site.
I have written a simple document that has how-to install and a minimum example.

Related issue number

closes #1500

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1532 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1532   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3793      3793           
  Branches       752       752           
=========================================
  Hits          3793      3793           

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