add requirements

doc/requirements.txt

+alabaster==0.7.12
+Babel==2.9.1
+certifi==2021.5.30
+chardet==4.0.0
+docutils==0.16
+idna==2.10
+imagesize==1.2.0
+Jinja2==3.0.1
+MarkupSafe==2.0.1
+packaging==20.9
+Pygments==2.9.0
+pyparsing==2.4.7
+pytz==2021.1
+requests==2.25.1
+six==1.16.0
+snowballstemmer==2.1.0
+Sphinx==4.0.2
+sphinx-rtd-theme==0.5.2
+sphinx-sitemap==2.2.0
+sphinxcontrib-applehelp==1.0.2
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==2.0.0
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.5
+urllib3==1.26.5