Architecture documentation

In this PR I will create an elaborate set of documentation regarding the internal workings and architecture of phpDocumentor. Due to the size of this effort it will probably take some time to be finished but I want to offer it as a WIP PR for everyone to see and review.

*Important* This PR contains information of changes found on the `develop` branch for phpDocumentor; which means that some of the features mentioned in this documentation will not be applicable for phpDocumentor2 but become available with phpDocumentor3.

TODO:

- [ ] Change plantuml sphinx integration to include external UML files
- [ ] Write a listing of available events and what they do
- [ ] Expand conventions document
- [ ] Review and expand doc on how descriptors are built
- [ ] Add missing chapters described in outline
- [ ] Review complete set of documentation