feat(docs): add dark/light theme toggle

[adxdits]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[x] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #5065

What is the new behavior?

• Added a theme toggle button in the navigation menu
• Users can switch between dark and light themes
• Theme preference is persisted in localStorage
• All components now use CSS custom properties for theming
• Light theme uses One Light color palette for syntax highlighting

Does this PR introduce a breaking change?

• Created ThemeService using Angular signals for reactive state management
• Defined CSS variables in :root (dark) and body.light-theme (light)
• Updated some components to use CSS variables instead of hardcoded colors

Other information

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[timdeschryver]

I like the light theme colors, nice!

[markostanimirovic]

Thanks @adxdits! 👑

[timdeschryver]

Great work @adxdits , thanks!