Add Shadow DOM examples

[pepelsbey]

⚠️ The same demos were already merged in mdn/interactive-examples#2707 but didn’t work as expected. Once these are merged, I’ll remove the previous ones.

Description

Adds declarative shadow DOM examples that can only work with {{EmbedGHLiveSample}} macros as remote demos.

Motivation

To support existing implementations in Chrome and Safari and for the upcoming one in Firefox 122 (behind the flag).

Related issues and pull requests

• Add a declarative shadow DOM example content#31577
• Update template element description and add shadowrootmode attribute content#31490

[bsmth]

Thank you!