feat: Update sidebar

[oodamien]

• Arrow on the left
• Distinguish page/non page (Resolves #236)

[philwebb]

I don't see a difference for the page/not page items in the tree. Here's a commit that updates the preview https://github.com/philwebb/antora-ui-spring/commit/2b1ff69dbecb839c8a8203b77c451804316b2aa7

[philwebb]

I wonder if we should also keep the chevron color the same as the text. It's the same when no selected, but when selected the text and chevron are different colors.

[philwebb]

When comparing to the stock Antora UI I think there are a couple of tweaks we should consider:

I think the stock version looks a little better because all items are left aligned (ours indents differently when the chevrons are present/missing). I also think the smaller chevrons look slightly better (but that's just cosmetic). docs.github.com use 16x16 icons which look pretty good.

[philwebb]

I'm a massive idiot and was looking at an old version of the code 🤕

If I use the actual current code things look good!