[FTF-328] Add a page for the Ably CLI

[m-hulbert]

Description

This PR adds a new section to the platform docs, under 'Tools' for the Ably CLI.

Checklist

• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch ftf-328-cli-user

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