feat(redesign): Alert component

[g-francesca]

Preview
This PR introduces a new Alert primitive for contextual callout blocks, replacing the legacy doc-box HTML pattern.

Changes

• New component — Alert.astro with three severity levels: info, alert, and warning, each with a distinct icon, color scheme, and optional auto-resolved title.
• Usage example — installing.mdx (renamed from .md) now uses the new Alert component for three different callout types, replacing the old doc-box div. Alers were added for demo purposes only.

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