[Doctrine] Set a canonical document for DoctrineBundle configuration reference

[phansys]

Configuration reference for DoctrineBundle is currently exposed from 2 documents:

1. Bundles > DoctrineBundle > Configuration Reference (defined at doctrine/DoctrineBundle);
2. Reference > Doctrine Configuration Reference (DoctrineBundle) (defined at this repo).

The first seems the more complete regarding the available configurations, although the second have some additional examples.

The proposal in this issue is to define a canonical document for the configuration, in order to avoid duplicates and to reduce the maintenance effort.

IMO, we should at least add link from [2] to [1].