Add Examples section with valid JSON & prose

[bsmth]

Page URL

https://firefox-admin-docs.mozilla.org/reference/policies/

What specific section or headline is this issue about?

No response

What did you expect to see?

The policy pages should have examples that are copy-paste ready with prose explanations.

Do you have anything more you want to share?

People copy-paste JSON and expect it to work, for example:

{
  "policies": {
    "DisplayBookmarksToolbar": "always" | "never" | "newtab"
  }
}

Where we should have:

The following `DisplayBookmarksToolbar` policy enforces that the Bookmarks toolbar is only displayed on new tabs:

```json
{
 "DisplayBookmarksToolbar": "newtab"
}
```

[bsmth]

CC @argl - would be nice to incorporate your recent changes here somehow.