[docs] simplify RTD config and use latest Sphinx (#3554)

* Update requirements_base.txt

* Update requirements_rtd.txt

docs/requirements_base.txt

-sphinx < 3.2
+sphinx
 sphinx_rtd_theme >= 0.3
 mock; python_version < '3'

docs/requirements_rtd.txt

-sphinx >= 3.0.2,<3.2
-sphinx_rtd_theme >= 0.5
-readthedocs-sphinx-ext > 2.1
-mock; python_version < '3'
-breathe
+-r requirements.txt
+sphinx >= 3.2.2