Bump sphinx from 2.0.1 to 2.1.0 in /docs (#75)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.0.1 to 2.1.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.0.1...v2.1.0)

Author: dependabot-preview[bot]

docs/requirements.txt

@@ -1,7 +1,7 @@
 # This file is used to setup env
 # to generate documentation.
 
-sphinx==2.0.1
+sphinx==2.1.0
 sphinx_autodoc_typehints==1.6.0
 sphinxcontrib-mermaid==0.3.1
 sphinx-typlog-theme==0.7.2