feat: support collapsible `example` blocks

[ghostsquad]

Similar to this:
https://golang.org/pkg/encoding/json/#HTMLEscape

Example

Example

<details>
<summary>Example</summary>

```jsonnet
this is an example
```
</details>

[tombrk]

Because docsonnet output is usually processed my a Markdown parser, above HTML snippet would work.

Agreed, there is probably a better syntax. Suggestions?

[ghostsquad]

I don't think there's better markdown compatible syntax