Docs: Update and write overviews for top sections

[iRaindrop]

Update or create the overview pages for each of the top sections (bucket/node) to reflect and accommodate the restructure. Also update or create overview pages for subsections as needed. An overview for subsection should be determined by the current thinking. If the subtopics are self-explanatory without an overview - that's better than a weak overview topic just to be consistent.

Here are the top sections per the restructure. Sections marked as update have legacy content in their _index.md files as overviews in the prior structure.

Section	Update	New
Getting Started		X
First Boot & Provisioning	X
OS Configuration	X
System Extensions	X
Deployments		X
Orchestration & Container Runtimes		X
Diagnostics & Fixing Issues	X
Nebraska Update Manager & Releases	X
Security		X
CoreOS Migration	X
Developer Guides	X
How to Contribute	X

All updates should replace the old title with the new one and provide an overview to the section - which will likely will be a complete rewrite for several of them.

Related subtasks:

Consolidate Concepts in Virtualization Options Overview: #578
Consolidate Concepts in Bare Metal Overview: #583
Consolidate Concepts in Cloud Providers Overview: #584
Consolidate Concepts in Virtual Machines Overview: #582