Renaming docs folder to doc

doc/developer/CMakeLists.txt

doc/developer/developer.xml

doc/user/CMakeLists.txt

doc/user/connectors.xml

doc/user/data-model.xml

doc/user/language-reference.xml

doc/user/persistence-architecture.xml

doc/user/server-administration.xml

doc/user/tutorial.xml

doc/www/content/doc/box-protocol.rst

doc/www/content/pages/documentation.rst

doc/www/pelicanconf.py

doc/www/plugins/documentation/documentation.py

extra/dist/tarantool.init