Add docs for the onboarding CLI tool

[amrit110]

This pull request adds a comprehensive command-line interface (CLI) reference for the aieng-platform-onboard tool, making it easier for users to understand installation, usage, options, onboarding steps, environment variables, and troubleshooting. The documentation is now included in the site navigation for improved discoverability.

Documentation improvements:

• Added a detailed cli.md file covering installation, CLI commands, onboarding workflow, options, environment variables, exit codes, examples, generated files, troubleshooting, and support information.

Site navigation:

• Updated mkdocs.yml to include the new CLI documentation in the navigation menu, making it accessible from the main docs site.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.88%. Comparing base (08c0cbd) to head (ed80690).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #26   +/-   ##
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  Coverage   82.88%   82.88%           
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  Files           3        3           
  Lines         409      409           
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  Hits          339      339           
  Misses         70       70           

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