CO-2650 CO-2735 Add custom data dictionary documentation.

[davehensley]

Pull Request/Issue Resolution

Description of Change:

I'm adding documentation for 3 new features that the Core Objects team has recently added to the custom data dictionary project:

• API access (read-only)
• Tag support
• CSV exports

Closes BD-3188

Is this change associated with a Braze feature/product release?

• Yes (currently in EA)
• No

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[josh-mccrowell-braze]

I know that this is in draft, and she is currently OOTO, but assigning this one to @lydia-xie for review when this one is ready to go! Just ping us when you're ready for a tech writer review.