Allow different sections to set the nav to be open or closed by default

[shainaraskas]

Some of the sidenavs are already very long, even when just H1s are visible (reference, deploy and manage, explore and analyze ...):

Some are quite short and could benefit from being open (manage data)

Propose being able to set an "expanded" / "collapsed" option for each section, which controls whether the H1s are open by default (and, as an extension of this, allow for collapse at H1)