[FEATURE] Link to the TYPO3 API (#678)

With this change you can link to any class etc on the phpapi:

```
*   :api-class:`\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer`
*   :api-class:`In main <api-dev:\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer>`
*   :api-class:`In 11.5 <api-11:\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer>
*   :api-class:`\TYPO3\CMS\Core\EventDispatcher\EventDispatcher <api:\TYPO3\CMS\Core\EventDispatcher\EventDispatcher>`
```

Author: linawolf

Documentation-rendertest/Api/Index.rst

@@ -0,0 +1,11 @@
+..  include:: /Includes.rst.txt
+..  _api:
+
+=========
+TYPO3 API
+=========
+
+*   :api-class:`\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer`
+*   :api-class:`In main <api-dev:\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer>`
+*   :api-class:`In 11.5 <api-11:\TYPO3\CMS\Extbase\Routing\ExtbasePluginEnhancer>
+*   :api-class:`\TYPO3\CMS\Core\EventDispatcher\EventDispatcher <api:\TYPO3\CMS\Core\EventDispatcher\EventDispatcher>`