docs(dotenv): improve dotenv docs

[kt3k]

This PR improves dotenv docs.

Some examples need the setup of actual .env files. Such examples are tagged as no-eval

part of #3764

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.70%. Comparing base (bd55f3b) to head (ca58a17).
Report is 3 commits behind head on main.

❗ Current head ca58a17 differs from pull request most recent head db6d62f

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5095      +/-   ##
==========================================
+ Coverage   92.57%   92.70%   +0.12%     
==========================================
  Files         481      482       +1     
  Lines       38701    38718      +17     
  Branches     5447     5462      +15     
==========================================
+ Hits        35828    35893      +65     
+ Misses       2818     2770      -48     
  Partials       55       55              

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[iuioiua]

Can you add a brief description explaining what this snippet is doing?

[iuioiua]

Nice!