Add support for handbook sub-site build to properly serve assets and images in development mode, and update relevant docs and handbook pages

What does this MR do?

Add support for handbook sub-site build to properly serve assets and images in development mode.

Also updates relevant docs and handbook pages around new commands/processes for running in development mode.

Note that you must navigate to http://localhost:4567/handbook, because there is no root index. This should be fixable with Middleman redirects (https://middlemanapp.com/basics/redirects/), so I didn't document this, but I couldn't get it to work immediately so I'm deferring that to a future MR.

Related issues

Relates #8302 (closed)