Restructure of top-level table of contents

This is something that's been on my radar for a while. I'd like to restructure the "NUnit" section of the docs to better reflect the different products it now spans. My thoughts:

• This structure predates the break up of the different projects, and reflects NUnit 2's technical structure, when there was less of a divide between the primary components.
• I want to make sure the docs to better separate out the framework, engine, console, NUnitLite runners, and NUnit Engine extensions. I'd hope this will make things clearer to end users that they are dealing with multiple products, of which they will likely be using at least two.
• I'd like to make the content more approachable for new users - highlight the 'every-day' stuff over the more advanced functionality.
• I'd like to better draw out the engine content, as it's own tool for users to work with.

I was thinking of looking at this after docfx was in place, but now I'm wondering if it's maybe better before. Reasons:

1. Looking at the current URL structure, it'll involve changing URLs. Docfx will handle the redirects, but given we're just about to change all the URLs anyway, it would be nice to avoid a second level of redirection.
2. Given this looks like it's going to be a bit of a "relaunch" for our docs, it would be good to get things structured in the right way from the start.

@SeanKilleen - what do you think? I can help with this, providing we can hold on till next weekend!

Start by reviewing the current NUnit top-level documentation structure and URL layout, then compare it with the requested separation of framework, engine, console, NUnitLite runners, and engine extensions. Define a revised navigation that emphasizes everyday user content and preserves appropriate redirects when DocFX is introduced. Done means the documentation structure and product boundaries are clearly reorganized.