Add option to allow expanding sidebar navigation

[casperdcl]

Can we have some sections expanded by default?

Maybe sidebar.json could support a new "expanded": true key next to anything that has "children": [...].

current	proposed

[yathomasi]

I also had a thought about this. I have noticed this while following through with documentation.
Few examples:

• https://nextjs.org/docs/getting-started
• https://www.gatsbyjs.com/docs/

Maybe sidebar.json could support a new "expanded": true key next to anything that has "children": [...].

Definitely, that would be an approach.