HTML guides

This first batch publishes 2 of the 11 upstream HTML guides; the rest are converted and will be released in later batches.

• Custom Elements — Define, register, and manage the lifecycle of autonomous custom elements, covering constructor rules, lifecycle callbacks, upgrade timing, attribute/property reflection, and naming conventions.
• Shadow DOM, Templates & Slots — Encapsulate a component’s DOM and styles with Shadow DOM, construct the tree efficiently with HTML templates, and project consumer content with slots.