[Build] Add heading component

[roadlittledawn]

Summary

Add heading component that supports specifying the level and ID attribute.

The ID attribute is important when users share links to specific sections of the site and for use by the On this Page ToC.

Currently, content authors primarily only use <h2> and <h3> headings, though I found at least two pages that use <h4>.

Acceptance criteria

• As a content author, I can add either a heading 2 or 3 element and optionally specify the ID attribute
• The ID attribute is automatically generated via a slug of the text of the element

Helpful links

Example: many h2 and h3
Example: h4 inside collapser

[roadlittledawn]

Closing this for now. We can add id attribute via markdown headings, so not sure we need these components right now. If there's something special we want to do with headings that markdown doesn't support we can re-open