API keys guide

[pabik]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  A guide about api keys to documentation

• Why was this change needed? (You can also link to an open issue here)
  A new feature was introduced with API keys, so a guide would be nice

• Other information:

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Name	Status	Preview	Comments	Updated (UTC)
docs-gpt	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 8, 2024 9:35am
nextra-docsgpt	✅ Ready (Inspect)	Visit Preview		Apr 8, 2024 9:35am

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 20.15%. Comparing base (6e147b3) to head (79ce8d6).
Report is 1 commits behind head on main.

❗ Current head 79ce8d6 differs from pull request most recent head db8db9f. Consider uploading reports for the commit db8db9f to get more accurate results

Additional details and impacted files

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##             main     #908   +/-   ##
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  Coverage   20.15%   20.15%           
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  Files          73       73           
  Lines        3315     3315           
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  Hits          668      668           
  Misses       2647     2647           

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