Fix up "index page" functionality

[ISSOtm]

• Fixes index.html fails to be created silently when there is no prefix chapter #1814 by only marking the first non-draft chapter as the "index" page, and adds a test for that
• Makes the link to the first chapter active in the "index" page, improving consistency (and making the aforementioned test pass without hacks)

Additionally, I would've liked to document that "index page" functionality, but I couldn't find any place for it in the documentation.

[ehuss]

I think this should be fine to land. It seems a little strange to not have a prefix chapter like an introduction. Another option would be to create an index.html that redirects to the first chapter, though that's probably not necessary.

[ehuss]

Thanks!