Add document capture introduction component

[aduth]

Relates to: LG-3251

Add DeviceContext context

Why: Since the design incorporates elements which vary depending on support for rear-facing ("environment") and front-facing ("user") video capture support, we need a means to be able to make this value available to all components of the application

Add documents introduction component

Why: While the introductory text is the same for desktop and mobile, on mobile devices it (1) occurs as its own standalone step and (2) includes one fewer instructions items (file size recommendation).

Screen Recording:

Note that the text "File size should be at least 2 MB" appears only if the user agent is not that of a mobile device.

Notes:

• The context was made generic to support future use-cases where front-facing camera capture may be supported on desktop devices. The distinction is not meant to be purely on mobile vs. desktop, but moreso on "supports direct capture" vs. "will rely on file uploads".
• There is intentional differences between the device support implemented here, and that of the Acuant Web SDK. Part of this is due to the fact that the Acuant SDK is very large and is loaded asynchronously, while we still need to be able to determine device as early as possible to present the user with relevant texts. Additionally, we don't want Acuant's reporting to be the sole indicator of "mobile", since (per design) we use this as a basis to direct the user to a mobile device. Since the user may be on an unsupported mobile device (based on Acuant's logic), we'd likely not want to direct the mobile user to "Use your phone".
• A PageHeading component was created with the sole purpose to abstract the implementation detail of consistency with current heading styling (using Basscss) in a way that can seamlessly be migrated to use Login Design System in the future.

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In Progress, To Do:

• Automated test cases for different device support flows (73ad86bb3)
• Localize introductory text (a18bc2dea)
• Styling (9b2a151)
• Inline documentation for useDeviceHasVideoFacingMode hook (4e51a9020)
• Automated testing for useDeviceHasVideoFacingMode hook (b8edf3fb9)
• Share screenshot or screen recording

[zachmargolis]

LGTM