Make @example a known doc tag

# Proposal

Add `@example` as a known tag with the following format

First line is the title (optional - if line is empty then there is no title)
Rest is the tag content

```cadl
/**
* @example First line is example title
* rest is
* example
* content
*
*/
```

woudl result in the following

```json5
{
title: "First line is example title",
content: "rest is\nexample\ncontent",
}
```