Replace plugin-generated index pages with Markdown

I realized I could replace plugin-generated index pages with Markdown files, which I hope will be easier to maintain, especially given the "Edit this page" links now in-place. All of the code changes in this PR are to remove the index page generator functions. I'll prepare a future PR to switch the team/ index page over; I've got a working commit in another branch, but I want to get some tests around it first.

@afeld May I bother you again for a code review? :-)
Done in all the Markdown files. Adjusted assets/_sass/_hub.scss.