Extract Sectioning components from 2U

Is your feature request related to a problem? Please describe.

Document outline is a feature of the HTML5 spec that has not been implemented by any major browser or a11y vendor. As such, content hierarchy is still primarily determined via heading rank.

In traditional web development, document outlines are managed by incrementing and decrementing heading levels to match the hierarchy of the content on the page. However, React apps are not built as documents, but rather via composition of components.

The component abstraction works best when it doesn't know about things outside of its API boundaries, like how it fits into a larger content hierarchy.

The 2U sectioning Readme has more explanation and examples on how components can achieve document outline and content hierarchy in a decoupled and composable manner.

Describe the solution you'd like

Extract and package 2U's sectioning components.

Describe alternatives you've considered

When authoring an HTML document, it is often times possible just manually count and increment/decrement heading levels. Additionally, the overall impact of document outline on SEO and a11y is debatable.

Additional context

The Readme is an MDX document, so it could probably be included as the package docs in hzcore, with caveats:
- The frontmatter would need to be adjusted to match hzcore docz hierarchy
peer dependencies:
- react
- react-dom

hzdg / hz-core