layout fixes for docs

Fixes:

* sidebar is now fixed to the left, and stays there as you scroll the page.
* on mobile: render the sidebar as a table-of-contents list, above the content
* tighten the navbar padding, as we get close to the mobile size, so that it does not get pushed below the logo
* fix accordion to open to the correct section on page load
* scroll to the correct anchor on page load, if we land here with an anchor hash 

still broken:

* accordion does not open to the right section when you use navigate forward or back, it stays stuck on the last section you visited