Add CLI usage documentation to README

[shekohex]

Fixes #226

Add documentation for the Tangle CLI usage in the cli/README.md file.

• Overview: Add an overview section at the top of the README.md file.
• Table of Contents: Add an index at the top of the README.md file.
• Installation: Document how to install the CLI from crates.io.
• Creating a New Blueprint/Gadget: Document how to use the CLI to create a new blueprint/gadget.
• Deploying the Blueprint: Document how to use the CLI to deploy the blueprint to a local Tangle node.
• Required Environment Variables: Document the required ENVs for the deployment.
• Examples: Provide examples with each step and the expected output.

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For more details, open the Copilot Workspace session.