Fix Jinja2 error on readthedocs builds

by requiring a Jinja2 version lower than 3.1.

- Also fix sphinx_requirements.txt to really only contain Sphinx stuff,
  readthedocs install is using setup.py install_requires anyway for synadm
  package install.
- This also fixes local docs builds!

sphinx_requirements.txt

@@ -1,9 +1,5 @@
-click
-requests
-pyyaml
-tabulate
-click-option-group>=0.5.2
-
-Sphinx==3.5.2
-sphinx-click==2.7.1
-sphinx-rtd-theme==0.5.2
+Jinja2<3.1
+Sphinx
+sphinx-click
+sphinx-rtd-theme
+myst-parser