Add erp5-doc.git submodule which contain some documentation bt5
SlapOS and NMS documentation are moved into bt and committed in erp5-doc.git. This will allow to install documentation in another erp5 and also to test them.
SlapOS and NMS documentation are moved into bt and committed in erp5-doc.git. This will allow to install documentation in another erp5 and also to test them.