Add Examples section to Introduction and add some renderings

[nigelmegitt]

DRAFT pull request for early review comments, to see if I'm heading in an agreeable direction.

Closes #553

• Add an Example Documents subsection to the Introduction
• Point to existing samples in Appendix E and Appendix I.2
• Add illustrative renderings of all the ISDs generated by each of those samples
• Improve the EBU-TT-D example's formatting and rendered appearance, using some additional styling features - fontFamily, color, linePadding.

Also updates the examples' ttp:contentProfiles values to use the IMSC 1.3 designator instead of the 1.2 designator.

[css-meeting-bot]

The Timed Text Working Group just discussed Add Examples section to Introduction and add some renderings w3c/imsc#632, and agreed to the following:

• SUMMARY: @nigelmegitt to add example, remove pointers in 2.1 to other examples, leave renders in place.

The full IRC log of that discussion

<nigel> Subtopic: Add Examples section to Introduction and add some renderings #632
<nigel> github: https://github.com//pull/632
<cpn> Nigel: It's a draft PR for now, to add examples to the Introduction, and add some renderings
<cpn> .. I want to check in before I do more on this
<cpn> .. It adds some orientation information for people unfamiliar with the spec. But want to pause to discuss
<cpn> Pierre: On the rendered examples, it's not a bad idea. But I'd rather we use actual examples. Those examples demonstrate the syntax but they're artificial
<cpn> ... Would it be possible to use a real example, and actual document or fragment?
<cpn> Nigel: I already have something like that in the BBC Subtitle Guidelines, so that's easy to do, but it takes up a lot of space
<nigel> -> https://www.bbc.co.uk/accessibility/forproducts/guides/subtitles/#Example-EBU-TT-D-document
<cpn> Pierre: Have a frame from a BBC show with the EBU-TT-D fragment, showing how it's used in practice?
<cpn> Nigel: Would that be useful?
<cpn> Pierre: I'm trying to think of something that's not been don elsewhere. MDN has examples. The IMSC test suite has exhaustive examples
<cpn> ... What's missing is how it shows up in practice
<cpn> Nigel: The BBC Subtitle Guidelines is a real world example
<cpn> ... [Shows the BBC example]
<cpn> Pierre: I think that's awesome. I'd just include this. The region stuff is helpful
<cpn> ... It could go in an Annex if it's too long
<cpn> ... Add an acknowledgement too
<cpn> Nigel: OK, yes
<cpn> Chris: Move it to an Annex?
<cpn> Nigel: For people reading from the top, and want orientation, I think it should go in the Introduction. Much easier than reading through the spec detail
<cpn> Pierre: I'm fine with putting it in the Introduction.
<cpn> Nigel: And leave the rendering examples in place in the PR?
<cpn> Pierre: That's fine, no strong opinion. Could link to them if in they're in the test suite
<nigel> SUMMARY: @nigelmegitt to add example, remove pointers in 2.1 to other examples, leave renders in place.

[nigelmegitt]

Example added as discussed, I think this is ready for review now @palemieux .