Add PANEL macro to visually group certain paragraphs

[ntrel]

I copied the CSS property values from already used values. The border-radius being 5px rather than 4px is copied from the message-box style. (I think the 4px value is for smaller items like buttons).

Use it for AssertExpression docs to group several 'implementation defined' paragraphs and an example together.
https://dlang.org/spec/expression.html#AssertExpression

[dlang-bot]

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[CyberShadow]

Nice