feat: Add general website embed support

[GeckoEidechse]

Renders

# Embeds

## Generic

{% embed url="http://example.com/" %}

{% embed url="https://github.com/" %}

{% embed url="https://northstar.thunderstore.io/package/northstar/Northstar/" %}

## Special cases

The following embeds are rendered in a special way, e.g. as an iframe

### YouTube

YouTube embeds are rendered as an iframe on GitBook, allowing to playback the video directly.

{% embed url="https://www.youtube.com/watch?v=dQw4w9WgXcQ" %}

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