fix(left-sidebar): unindent nested details

[caugner]

Description

Updates the left-sidebar layout, unindenting nested details:

• If the details element has nested details, unindent (only) the summary element.
• If the details element has no nested details, unindent the whole details element.

Motivation

1. Avoid confusion.
2. Save horizontal space (possibly less line breaks in nested sidebars).

Additional details

Sidebar	Before	After
CSS
WebExt

Related issues and pull requests

Fixes #1311.

[github-actions]

01dd10c was deployed to: https://fred-pr1312.review.mdn.allizom.net/

[caugner]

At first, I thought the new behavior for CSS properties is confusing, but it's actually consistent with the current behavior at the top level, so I think it's fine.

[LeoMcA]

I think this is overly complex, and piles on the edge-cases - a path we already know to be dangerous with the sidebars.

I'd first take a step back and re-evaluate how we consider the sidebar hierarchy, which I've attempted to do in #1311 (comment). We can then revisit and fix some remaining edge-cases with the not-yet-baseline-green :has selector - it shouldn't be the basis of our whole hierocratical display.