Reintroduce support for docs-only and docs-only+ sites

[chalin]

I've suspected that docsy-only support wasn't working, but I've run out of time to retest the feature. Well, @adlerzei has been confirming that it's broken (thanks!). This is a tracker/meta issue for reintroducing support for:

• docsy-only sites
• docsy-only+ sites, that include a blog, etc

Tasks

• Setup a test site
• ...

Related

• Pages missing in the section index of docs-only sites #2499
• Top-level section landing pages are always rooted in a docs-only site #2492