Adds USPs to spaces docs

[franrob-projects]

This PR:

• Adds temporary USP components.
• Adds short statements within USP components that direct the user to supporting evidence in Ably's architecture docs.

Note that once the copy has been confirmed, I will then add the finished components designed by Lenker.

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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