Issue 955

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #955
| Included PRs | #2734

I wanted to fix #955 and it turns out that a complete rewrite was better. It uses the same structure as the console documentation: helper docs got their own articles and one big article talking about the other functionality.

### TODO

From the things ryan gave in #955:

* [ ] details about `EngineInterface`
* [x] helpers
* [ ] globals
* [ ] caching
* [x] AssetHelper
* [x] output escaping

@weaverryan, what do you mean with (1)? A doc about how to create your own engine?

### The Future

The `cookbook/templating/PHP` doc is now somewhat equal to this one. In the future, we should do something like what #2545 does with `book/translation`.