[a11y] Links in sidebar should use headings syntax, not just style

[alanvww]

→ Describe the issue, including on which page it can be found
For the current sidebar, all section titles are enclosed in "**{title}**", which renders the content only in bold, but not as headings, creating accessibility issues for those using screen readers.

→ Screenshots or relevant documentation

→ Example snippet causing the issue

- **ml5 Models**

- [Bodypose](/reference/bodypose.md)
- [BodySegmentation](/reference/body-segmentation.md)
- [Handpose](/reference/handpose.md)
- [Facemesh](/reference/facemesh.md)
- [ImageClassifier](/reference/image-classifier.md)
- [Sentiment](/reference/sentiment.md)

→ Potential solution and a11y standard it will meet (AA/AAA)

- ## **ml5 Models**  <- render as h2

- [Bodypose](/reference/bodypose.md)
- [BodySegmentation](/reference/body-segmentation.md)
- [Handpose](/reference/handpose.md)
- [Facemesh](/reference/facemesh.md)
- [ImageClassifier](/reference/image-classifier.md)
- [Sentiment](/reference/sentiment.md)

→ Low priority? (Y/N)
N