Cleaning up the 'ref' folder

I've cleaned up the 'ref' folder, which contained several outdated pages. Notably:

The old [project structure](http://framework.zend.com/manual/2.3/en/ref/project.structure.html) page was showing the file structure of a ZF1 app and module. I've updated this to show a typical ZF2 app (basically the skeleton app) and added some short descriptions. I've removed the module structure section and linked to the Module Manager section instead (which covers this in detail). I've removed the info on URL rewriting, which I don't think belongs here.

I've rewritten the old [installation](http://framework.zend.com/manual/2.3/en/ref/installation.html) page, by lifting some content from the skeleton app README and merging in some of the URL rewriting instructions from the page above. Some of this content duplicates instructions we have in the 'getting started' tutorial, but I think it's good to have a dedicated installation page.

I've deleted the following pages we had on performance which were all ZF1 specific:

* http://framework.zend.com/manual/2.3/en/ref/performance.classloading.html
* http://framework.zend.com/manual/2.3/en/ref/performance.database.html
* http://framework.zend.com/manual/2.3/en/ref/performance.introduction.html
* http://framework.zend.com/manual/2.3/en/ref/performance.localization.html
* http://framework.zend.com/manual/2.3/en/ref/performance.view.html

And I've removed the 'requirements' pages which also looked ZF1-specific, and had formatting broken to the point of being unreadable:

* http://framework.zend.com/manual/2.3/en/ref/requirements.dependencies.table.html
* http://framework.zend.com/manual/2.3/en/ref/requirements.php.extensions.table.html
* http://framework.zend.com/manual/2.3/en/ref/requirements.html
* http://framework.zend.com/manual/2.3/en/ref/requirements.zendcomponents.table.html