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.

/reviewed-on !875

.gitmodules

+[submodule "doc-bt5"]
+	path = doc-bt5
+	url = https://lab.nexedi.com/nexedi/erp5-doc.git
+	branch = master

bt5/erp5_slapos_tutorial

+../doc-bt5/erp5_slapos_tutorial
\ No newline at end of file

bt5/erp5_slapos_tutorial_data

+../doc-bt5/erp5_slapos_tutorial_data
\ No newline at end of file

doc-bt5 @ c605342f

+Subproject commit c605342f0c1546868552c29441ddf684b374d5d1