Separate use-cases, quick start pages: (asciidoctor#652) (asciidoctor#654)

[sturtison]

Separate use-cases and quick start pages: #652 #654

Create separate pages from existing content into:

• use-cases.adoc
• quickstart.adoc
• diagrams-extension-quickstart.adoc

Update nav.adoc.

Resulting navigation:

[sturtison]

Content moved, links and nav adjusted. Some content unchanged but rearranged. e.g. move "you will learn" to the top of the page.

[sturtison]

After:

[ggrossetie]

Thanks!

[mojavelinux]

I like the idea of introducing more pages so that each page is more focused and digestible.

I would like to point out that the page titles now have inconsistent capitalization. Could we agree on a style and stick to it. See https://docs.asciidoctor.org/browser-extension/diagrams-extension-quickstart/ Notice "Extension Options" verse "Diagrams extension quickstart". In the core documentation, we tend to use title capitalization for page titles. See https://docs.asciidoctor.org/asciidoctor/latest/