[Navigation] Categorisation is weak

[Dave Stewart]

I've been using Semantic UI for about 2 months now, and I still have to do a lot of hunting around to find things.

I think the various elements could be recategorised to make it more obvious:

• Things like Rail, Modal, Segment, Grid, Card should be moved to something like "Layout"
• Form elements should go in their own "Forms" section

I may knock up a demo then a PR to suggest.