📝 Add example code for sa_column onupdate timestamps

[RobertRosca]

Issue #370 is a common usecase and there have been a few other questions about how to implement created_at or updated_at timestamps in rows. This PR adds examples to the advanced section of the documentation on how this can be done with sa_columns, and also links the relevant sections of Pydantic documentation which show how to use factories or validators to achieve a similar thing with Pydantic.

[codecov]

Codecov Report

Base: 98.49% // Head: 97.74% // Decreases project coverage by -0.75% ⚠️

Coverage data is based on head (d78fc71) compared to base (ea79c47).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head d78fc71 differs from pull request most recent head b160fc8. Consider uploading reports for the commit b160fc8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
- Coverage   98.49%   97.74%   -0.76%     
==========================================
  Files         185      186       +1     
  Lines        5856     5901      +45     
==========================================
  Hits         5768     5768              
- Misses         88      133      +45     

Impacted Files	Coverage Δ
docs_src/advanced/sa_column/tutorial001.py	0.00% <0.00%> (ø)

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[github-actions]

📝 Docs preview for commit 893bcf3 at: https://63131ab0d8aeff704c33fd2d--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit 1e5caaa at: https://6326cad9fcbd027752da81e2--sqlmodel.netlify.app

[github-actions]

📝 Docs preview for commit d78fc71 at: https://636244bbeb67cb7064300959--sqlmodel.netlify.app

[RobertRosca]

Full path to the docs page: https://636244bbeb67cb7064300959--sqlmodel.netlify.app/advanced/sa-column/

[github-actions]

📝 Docs preview for commit b160fc8 at: https://639ce08a1f184e006c3a2967--sqlmodel.netlify.app

[MRigal]

This is actually very valuable documentation which I was a bit missing. What is it missing to get merged?