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 --- #### 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)