Overhaul the "Getting started" section

[adamralph]

Including, but not limited to, addressing these points:

• The "overview" shouldn't be about just NServiceBus. This is an overview page in the top level "Getting started" section, so it's in the context of the platform, not just NServiceBus
• We have both an "overview" and "High-level content" pages—it feels like these should be consolidated
• The "overview" and "High-level content" pages appear after the sub-sections of the top level "Getting started" section in the menu. This seems at odds with other sections, which usually have an "Introduction" (or similar) page at the top of the menu section.
• The four links in the "overview" don't match, and don't even include, links to the subsections in the menu