Remember which panels are open or closed in the nav

[Fil]

does it need a cookie? see how vitepress does this, maybe.

[mbostock]

Related #159 which covers the more specific bug where you visit a page and the corresponding section is not automatically opened, which only occurs when the open option is set to false in the config.

[mbostock]

We should use localStorage for this rather than a cookie, like we do for the sidebar toggle.

https://github.com/observablehq/cli/blob/aca06665cae3c2a64cfb502112c7ecb691284c54/public/client.js#L316-L317