Add "Insights" section for explanation docs

[behoppe]

Issues
The documentation page has places for tutorials, user's guide, and reference. But it has no place for explanation. It needs an explicit place.

Improvements
Let's add a section called "Insights." (Thanks @DorothyCurtis for the name suggestion.)

Additional discussion

It seems likely that some articles will qualify both as "insights" and as blog posts. Probably we want to help users find these articles, without requiring that they guess correctly how we have categorized them. Tags could help. We might associate a tag with every category, and use the "insights" tag on blog posts that qualify.