feat: support github style alerts

[imfing]

Support GitHub style alerts.

Implemented GitHub alerts as per #460. Future PRs should aim to support the extended syntax from Obsidian, which is a lower priority.

Example:

> [!NOTE]
> Useful information that users should know, even when skimming content.

> [!TIP]
> Helpful advice for doing things better or more easily.

> [!IMPORTANT]
> Key information users need to know to achieve their goal.

> [!WARNING]
> Urgent info that needs immediate user attention to avoid problems.

> [!CAUTION]
> Advises about risks or negative outcomes of certain actions.

Output:

[netlify]

✅ Deploy Preview for hugo-hextra ready!

Name	Link
🔨 Latest commit	621f979
🔍 Latest deploy log	https://app.netlify.com/sites/hugo-hextra/deploys/675e177d21ab9c0008221a8a
😎 Deploy Preview	https://deploy-preview-513--hugo-hextra.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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