auto expand toctree sections

[eharkins]

toctrees define the expandable sections in the sidebar navigation on read the docs. I think it's true in general that we would like these to be expanded by default to reveal the entries within them, rather than them being hidden by default.

See nextstrain/ncov#709 (comment), nextstrain/ncov#709 (comment)

[tsibley]

This will be addressed (at least partially), by the change for #8. We'd want to expand only one-level deep, yeah?

[eharkins]

We'd want to expand only one-level deep

That sounds good as a starting point / default.

[jameshadfield]

This issue has been addressed for the main docs project but not yet for the subprojects (e.g. ncov). @tsibley was this achieved by nextstrain/docs.nextstrain.org@7533335 ?

[tsibley]

@jameshadfield Yep.