style: improve sidebar item layout, focus style, and spacing

[MohamedH1998]

Summary

What changed

• Override on the SidebarSublist - we were doing a lot of hacky things to customise this component, it made more sense to just create an override and create the compont ourselves
• Aligned the styling closer to what we're seeing in the dashboard
• Focus ring — added keyboard-only focus styles (focus-visible:outline-blue-500 / dark:focus-visible:outline-blue-400) to sidebar links and group toggles, matching the existing pattern used on the sidebar search input; outline-offset-0 keeps the ring flush to the element so it does not overlap adjacent items
• Focus ring clip fix — added padding-top: 1px to .sidebar-content to match the gap value, preventing the focus outline on the first sidebar item from being clipped
• Sidebar width — decreased the width; narrower than the original Starlight default (300px)

Screenshots (optional)

Before:

Screen.Recording.2026-03-18.at.12.03.33.mov

After:

Screen.Recording.2026-03-18.at.12.15.15.mov

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
*.ts	@cloudflare/content-engineering, @kodster28
*.astro	@cloudflare/content-engineering, @kodster28
*	@cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://706d94c7.preview.developers.cloudflare.com
Preview Branch URL: https://style-sidebar-improvements.preview.developers.cloudflare.com