[WIP] The Testing Documentation section is unclear.

I installed `Sphinx` then did the `git submodules` next generated the docs thus skipping the `bash install.sh`

```
* Install `Sphinx`_;
* Install the Sphinx extensions using git submodules: ``git submodule update --init``;
* (Optionally) Install the bundle docs and CMF docs: ``bash install.sh``;
* Run ``make html`` and view the generated HTML in the ``build`` directory.
```

It is unclear what to expect after each step.

Opening ie `$ open _build/html/cookbook/controller/error_pages.html` it does not look at all similar to http://symfony.com/doc/current/cookbook/controller/error_pages.html

So I ran `bash install.sh` expecting that should do it. Nope.

I would propose the following changes:

- [ ] Add text about test sphinx is already installed.
- [ ] We should explain how the build will look like.
- [ ] We should explain why `bash install.sh` is optional.
- [ ] Tell about the additional _ext directory and how not to add it to the PR.