Nav Sidebar: move to left hand side "W" menu and use "dark UI"

[getdave]

In #39290 we introduced the Navigation Sidebar as a dedicated UI to expose the Navigation structure outside of the block.

As per #36667 we should now look to move this sidebar to the left hand sidebar and use the "dark" UI.

The latest prototype for this is in the following Figma file.

design.mp4

[getdave]

@javierarce @SaxonF If we have more up to date designs for this now then would it be possible for you to update the Issue description and link up any Figma files? Much appreciated.

[annezazu]

Is this still relevant or can we close it out with the navigation added? #46436

[getdave]

Seems like it's completed as part of the addition of Navigation to the Browse Mode for Gutenberg Plugin (though not in WP 6.2).