Restore the requiremnt file for generating the doc.

Used the following command:
pipenv lock --requirements --dev > docs/requirements.txt

Author: Ezibenroc

docs/requirements.txt

@@ -0,0 +1,29 @@
+-i https://pypi.org/simple
+attrs==18.1.0
+backcall==0.1.0
+certifi==2018.4.16
+chardet==3.0.4
+coverage==4.5.1
+cython==0.28.3
+decorator==4.3.0
+hypothesis==3.57.0
+idna==2.6
+ipython-genutils==0.2.0
+ipython==6.4.0
+jedi==0.12.0
+parso==0.2.1
+pexpect==4.6.0; sys_platform != 'win32'
+pickleshare==0.7.4
+pkginfo==1.4.2
+prompt-toolkit==1.0.15
+ptyprocess==0.5.2
+pygments==2.2.0
+requests-toolbelt==0.8.0
+requests==2.18.4
+simplegeneric==0.8.1
+six==1.11.0
+tqdm==4.23.4
+traitlets==4.3.2
+twine==1.11.0
+urllib3==1.22
+wcwidth==0.1.7