Restructure use cases section

[chrisn]

This change reorganises the Use cases section to have a subsection per use case, rather than organising by event triggering, rendering of web resources, and rendering of embedded web resources.

---

Preview | Diff

[chrisn]

I have finished working on this for now. Aside from restructuring as mentioned above, I have:

• Added Audio stream with titles and images use case, covering both HLS timed metadata and RadioVIS.
• Moved content from Rendering of Web content embedded in media containers to introductory paragraph while retaining the three examples. We could move these into their own subsections if we want to add detail.

@gmandyam, please take a look, thanks!