Add dark/light theme toggle to docs.layer5.io

### Current Behavior
The Layer5 Docs website (https://docs.layer5.io) does not have a dark/light theme toggle option.

Other Layer5 websites like the Kanvas page already include this feature.

### Desired Behavior
Add a dark/light theme toggle to the Layer5 Docs website so users can switch between light mode and dark mode.

### Implementation
1.Add a theme toggle button in the navbar/header.
2.Reuse the existing theme toggle design from other Layer5 websites if possible.

### Acceptance Tests
1.Theme toggle is visible in the navbar.
2.Users can switch between dark and light themes.
3.Works properly on desktop and mobile.

### Mockups
Current Docs Website (No theme toggle)

https://github.com/user-attachments/assets/5e77fc76-2216-49f5-930a-44ad1f1aeee9

Reference from Kanvas Website (Theme toggle available)

https://github.com/user-attachments/assets/277f6910-f353-4f45-bf4d-09791fe19feb

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#### Contributor Guide and Resources
- 📚 [Instructions for contributing to documentation](https://github.com/layer5io/docs/blob/master/CONTRIBUTING.md)
- Layer5 documentation [site](https://docs.layer5.io) and [source](https://github.com/layer5io/docs/)
- 🎨 Wireframes and [designs for Layer5 site](https://www.figma.com/file/5ZwEkSJwUPitURD59YHMEN/Layer5-Designs) in Figma [(open invite)](https://www.figma.com/team_invite/redeem/GvB8SudhEOoq3JOvoLaoMs)
- 🙋🏾🙋🏼 Questions: [Layer5 Discussion Forum](https://discuss.layer5.io) and [Layer5 Community Slack](http://slack.layer5.io)