Document the Ace Editor

[Michael-F-Bryan]

#338 added the ace editor but we didn't really document it in the user guide. There are currently two parts to making code snippets editable, first you need to enable editing in your book.toml, then you need to add an editable annotation to your code blocks.

The config file:

[output.html.playpen]
editor = "./path/to/ace/editor"  # defaults to using the ace editor
editable = false # defaults to false

Making a specific code snippet editable

```rust,editable
use std::fs::File;
File::open("foo.txt").unwrap();
```