doc: update doc/sphinx/requirements.txt

Pin all dependencies as reported by 'pip freeze'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/sphinx/requirements.txt

+alabaster==0.7.12
+Babel==2.9.1
+certifi==2021.10.8
+charset-normalizer==2.0.12
 docutils==0.16
-sphinx==3.4.3
-sphinx_rtd_theme==1.0.0
+idna==3.3
+imagesize==1.3.0
+Jinja2==3.0.3
+MarkupSafe==2.1.1
+packaging==21.3
+Pygments==2.11.2
+pyparsing==3.0.7
+pytz==2022.1
+requests==2.27.1
 six==1.16.0
+snowballstemmer==2.2.0
+Sphinx==3.4.3
+sphinx-rtd-theme==1.0.0
+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==1.26.9