Flatten nested sidebar items

[dundalek]

When there is a single namespace the nested hierarchy is not useful and takes extra clicks to expand.

For example instead of

com >
  org >
    project >
      foo
      bar

we could flatten as

com.org.project >
  foo
  bar

Is there also a way to perhaps have some items in sidebar expanded by default? With some heuristic to expand e.g. if there are less than 10 children items.

Collapsible categories are collapsed by default. If you want them to be expanded on the first render, you can set collapsed to false:
https://docusaurus.io/docs/sidebar/items#expanded-categories-by-default