Introduction: include an example annotated document and render

[nigelmegitt]

In the introduction, it would be really helpful to include an example pair of documents, one Text and one Image profile, that demonstrate what IMSC can be used for, and the kind of expected output. Ideally they would be annotated.

Originally posted by @nigelmegitt in #552

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Update: assuming we close #600 by merging #603 the scope of this will reduce to just one Text profile document

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Update: changed scope to single example, since the document is now Text Profile only.

[palemieux]

@nigelmegitt Annex E already contains examples. Would a sample render of both resolve your issue?

https://www.w3.org/TR/ttml-imsc1.2/#sample-instance

[nigelmegitt]

@palemieux yes, though for preference I would add some styling to the Text profile example, and not just in the region - for example a different color and background color applied to the text content by putting it into spans.

[palemieux]

There are also the existing MDN pages.