Add some examples to Shadow DOM Styling page

[rolfsmeds]

The docs currently mention shadow-specific selectors like :host and ::slotted, but doesn't show how to use those. It would be good with some code examples of stuff like

• Combining :host() with classnames and attributes.
• Using ::slotted() and its limitations
• Part name attribute selectors
• General limitations (e.g. lack of parent context)

(Also move the "No longer recommended" admonition higher up to be more prominent.)

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