[Docs] Mobile sidebar does not preserve navigation hierarchy

### Current State
On desktop, the sidebar clearly shows the navigation hierarchy by indenting the subsections under their main sections.

On mobile, all navigation items appear at the same level, making it difficult to distinguish the main sections from their subsections.

### Desired State

The mobile sidebar should show the same navigation structure as the desktop sidebar, making it easier to understand and navigate.

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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)