Add instructions for secrets in CLI

[bryannho]

• Added Defining environment variables section in cli.md to document support for secrets. Instructs users on how to configure secrets using .env file and deploy the project. Also links to env_vars.md for instructions on how to access and verify secrets from their application
• Added a note in env_vars.md that directs users to cli.md if they want to configure environment variables there

Related PR: https://github.com/ploomber/cli/pull/33

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📚 Documentation preview 📚: https://ploomber-doc--98.org.readthedocs.build/en/98/

[bryannho]

@edublancas added some documentation on secrets in cli.md. There was already sample code in env_vars.md that checks if a given env var is defined and returns a response, so I just linked to it. The docs build is giving errors when linking to specific sections in other pages, so I just linked to the other page as a whole. Let me know if it's clear enough