doc navigation - fixed header, sidebar?

[Mike Bayer]

I've always been frustrated with how you can't figure out where you are when you're deep into a documentation page. Can we try a layout like this page: https://foundationdb.com/layers/sql/documentation/GettingStarted/SQLAlchemy where:

1. our yellow box at the top with "prev/next" etc. stays fixed as it hits the top of the page

2. the sidebar is an independent scrollable bar with dynamic highlighting

3. we move "search" somewhere so that it at least is always on the page

[Mike Bayer]

after many days of experimenting, getting it to work, realizing that anchors don't work so revisiting the whole thing again, this is now up on 0.7-1.0.