Add a line border to show hierarchy better in main navigation

[cderv]

A border might help:

.sidebar-menu-container .sidebar-section {
  border-left: 1px solid lightgrey;
}

Originally posted by @jimjam-slam in #9777 (comment)

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This would be like we do in TOC right sidebar

[mcanouil]

Minor note "lightgrey" should be SASS variable to match the chosen theme.

[jimjam-slam]

border-inline-start may also be more appropriate than border-left if you need RTL support!

[cscheid]

border-inline-start may also be more appropriate than border-left if you need RTL support!

That's a whole different can of worms, I fear.

For example, I don't know if our foldable menu chevrons work well in RTL, etc. I'm not sure which is worse:

• make it very obvious that this won't work well in RTL by using border-left
• make it work only halfway with RTL

I tend to err on the side of "if we're going to do it, we need to do it very very well", and so I'd argue for border-left instead.

[jimjam-slam]

That's fair! I'm not as across RTL as I ought to be myself, and I can see that mixing systems is probably also a bad idea.