Add docs for bake

This can surely be improved upon - but should be a decent start.

Closes https://github.com/cakephp/cakephp/issues/5203
They are, but the meaning here is adding to:

```
Available bake commands:

- all
- xyz
```

(should probably say commands then)

I don't think it's logical to title the section "Creating new bake tasks" when it's obvious until you read the section that that is how bake's available commands are defined.