Pin sphinx version (fix for current docs build errors) (#1382)

* Update requirements.txt

* Update docs/requirements.txt

Co-Authored-By: Jirka Borovec <Borda@users.noreply.github.com>

* freeze sphinx version

* Update docs/source/conf.py

Co-Authored-By: Adrian Wälchli <aedu.waelchli@gmail.com>

Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>
Co-authored-by: J. Borovec <jirka.borovec@seznam.cz>

docs/requirements.txt

@@ -9,4 +9,4 @@ sphinxcontrib-mockautodoc
 git+https://github.com/PytorchLightning/lightning_sphinx_theme.git
 # pip_shims
 sphinx-autodoc-typehints
-sphinx-paramlinks
+sphinx-paramlinks