feat(docs): add dark mode support

[Amarprasad33]

Summary

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tambo-dark.mode.mp4

This PR enables full dark mode support for the documentation site, including a theme switcher, theme-aware icons, and appropriate styling updates.

• Added ThemeProvider to handle theme context and switching.
• Enabled the theme toggle in the main layout configuration.
• Implemented responsive icons that automatically switch assets based on the active theme.
• Updated styling with CSS variables for better dark mode visibility (borders, validation).

Key Changes

Docs (docs/src)

• layout.tsx: Wrapped application in ThemeProvider (next-themes) to enable theme persistence and switching.
• layout.config.tsx: Enabled themeSwitch in the baseOptions configuration.
• components/icons.tsx: Created TamboLogoIcon and TamboOctoIcon components that dynamically render Light or Dark SVG assets based on resolvedTheme.
• (docs)/styles.css: Added --color-dm-nav-border and --color-lm-nav-border variables for theme-aware navigation borders.
• Assets: Added Octo-Icon-Dark.svg and Tambo-Lockup-Dark.svg to public/logo.

Test plan

• Open the documentation and verify the theme switch appears in the header.
• Toggle between Light and Dark modes; ensure background and text colors invert correctly.
• Verify that the Tambo Logo and Octo Icon switch to their Dark variants when Dark mode is active.
• Check navigation borders and ensure they are visible and styled correctly in both modes.
• Refresh the page to ensure the selected theme persists.

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