docs(Plex): add plex, font-face documentation

[tw15egan]

Closes #9370

Adds documentation informing users that IBM/Plex will be emitted by default, and how to configure this feature. Also notes how to emit additional typefaces

Changelog

New

• _font-face.scss documentation in @carbon/styles

Changed

• css--font-face documentation in migration docs

Testing / Reviewing

Ensure docs make sense and are correct

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