Bump sphinx-rtd-theme from 0.5.0 to 0.5.1

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 0.5.0 to 0.5.1.
- [Release notes](https://github.com/readthedocs/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.5.0...0.5.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/docs.txt

@@ -8,7 +8,6 @@ alabaster==0.7.12         # via sphinx
 babel==2.8.0              # via sphinx
 certifi==2020.6.20        # via requests
 chardet==3.0.4            # via requests
-colorama==0.4.4           # via sphinx
 docutils==0.16            # via sphinx
 idna==2.10                # via requests
 imagesize==1.2.0          # via sphinx
@@ -21,7 +20,7 @@ pytz==2020.4              # via babel
 requests==2.24.0          # via sphinx
 six==1.15.0               # via packaging
 snowballstemmer==2.0.0    # via sphinx
-sphinx-rtd-theme==0.5.0   # via -r requirements/docs.in
+sphinx-rtd-theme==0.5.1   # via -r requirements/docs.in
 sphinx==3.3.0             # via sphinx-rtd-theme
 sphinxcontrib-applehelp==1.0.2  # via sphinx
 sphinxcontrib-devhelp==1.0.2  # via sphinx