feat: Update Docs theme to match brand

[easymac]

New:

Old:

Description

Updates the Docs theme (based on Starlight Nova.)

• Makes the navigation sidebar expanded by default to make the structure of our documentation clear and immediately scannable
• Replaces the old teal/cyan highlights with white, appearing visually cleaner and more contemporary
• Integrates more styled components (buttons, cards), making the possible next steps on a given page more visually prominent
• Replaces the unstyled contact us links with styled cards
• Integrates the new branding's primary font with a slightly tighter, more confident type hierarchy, which gives the pages a more premium feel

The overall effect is a shift from functional documentation to a considered part of the product experience, better suited to the sophistication of the Shorebird platform.

Also, it uses View Transitions to animate between the light and dark theme which is really cool 🔥 🔥

[dawn-ducky]

Looks good!