Improve left navigation by highlighting active section

[KOTungseth]

Description

In the AsciiDoc docs, the left nav highlights the active section with a gray box. This makes it easy for users to quickly identify where they are within the docs structure.

In v3, there is no clear visual indicator for the active section. This makes it harder for users to orient themselves and navigate between related pages.

Proposed solution

• To indicate the active section in the v3 left nav, implement a highlight style similar to the gray box in the AsciiDoc docs.
• Ensure the highlight is consistent across all v3 docs pages.
• Confirm the style aligns with the overall visual design of v3. For example, accessible contrast and consistent spacing.

Acceptance criteria

• The active section in the v3 left nav is visually distinct/highlighted.
• The highlight persists when navigating between pages within the same section.
• The design is accessible and matches the visual language of v3.