split off virtualbox networking into separate page

v2/networking/index.html.md encourages the reader to seek
provider-specific information under the documentation for the provider,
so for consistency, any virtualbox-specific networking info should be
placed there, not in the general networking section.
use monospace for technical terms where appropriate
eliminate slightly confusing wording

It doesn't make sense to use the present tense when saying box files are
split into two components, when one of those components is the box file
and one is something else.

One better way of phrasing it would be to use the past tense: "Box files
were split into two separate components" but even that's not completely
correct, because the old format did not include the metadata JSON
document which is one of the new components.

So it's safer to just say that today there are two different components.
reverse order of two box components

The metadata is optional whereas the box file is required, so it makes
sense to list the box file first.  It's also easier on the reader's
brain to start with the more obvious and easily understandable item
(which they'll probably be expecting to read about anyway) and save
the more surprising item till later.
distinguish more clearly between two types of box metadata

There are two types of box metadata, both in JSON format, so
we need to minimise the potential for confusion between them.
Renaming the component outside the box file to include the
word "catalog" makes it clear that this JSON document can
catalog potentially multiple boxes in one go.
mention that the "url" can also be a file path
Merge pull request #3618 from aspiers/box-format-doc-improvements

website/docs: box format doc improvements
Merge pull request #3616 from aspiers/virtualbox-networking-doc

website/docs: split off virtualbox networking into separate page
website/docs: document NAT interface [GH-3619]
core: Show message when checksumming box [GH-3623]