docs: reorganize

[tmorrell]

:heart: Thank you for your contribution!

Description

Reorganize the docs so they favor the REST API instead of the MDS API. With 4.5 this will close #72

Checklist

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• [x] I've created logical separate commits and followed the commit message format.
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• [x] I've added relevant documentation.
• [x] I've marked translation strings.
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[tmorrell]

Also will close #95

[tmorrell]

Tests should pass once https://github.com/inveniosoftware/datacite/pull/94 is merged