Restructure Platform navigation

[ftisiot]

What should we add to our content collections?

Following the comments on #953, we should restructure the navigation of the Platform (under /docs/platform) content. One possible option, as @lornajane suggested would be to

instead of top-level Concepts and HowTo, do another level of topics/groups under the Platform section before splitting to concepts/howto

Is this something you would work on yourself?

[ ] I plan to open a pull request for this