feat: enable arbitrary code snippets in docs

[abvthecity]

Similar to <Markdown src="../path/to/markdown.ts" />, support code snippets like <Code src="../path/to/code.java" />

This can be combined with <CodeGroup> like this:

<CodeGroup>

<Code src="snippet.ts" />
<Code src="snippet.py" />
<Code src="snippet.go" />
<Code src="snippet.java" />

```plaintext title="Arbitrary Code"
hello world!
```

</CodeGroup>

Fixes FER-2579

[dsinghvi]

sick feature!