Bump sphinx-rtd-theme from 0.5.0 to 0.5.1 in /requirements (#16255)

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](https://github.com/readthedocs/sphinx_rtd_theme/compare/0.5.0...0.5.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/docs.txt

@@ -86,9 +86,9 @@ sphinxcontrib-htmlhelp==1.0.3 \
 sphinxcontrib-qthelp==1.0.3 \
     --hash=sha256:4c33767ee058b70dba89a6fc5c1892c0d57a54be67ddd3e7875a18d14cba5a72 \
     --hash=sha256:bd9fc24bcb748a8d51fd4ecaade681350aa63009a347a8c14e637895444dfab6
-sphinx-rtd-theme==0.5.0 \
-    --hash=sha256:373413d0f82425aaa28fb288009bf0d0964711d347763af2f1b65cafcb028c82 \
-    --hash=sha256:22c795ba2832a169ca301cd0a083f7a434e09c538c70beb42782c073651b707d
+sphinx-rtd-theme==0.5.1 \
+    --hash=sha256:fa6bebd5ab9a73da8e102509a86f3fcc36dec04a0b52ea80e5a033b2aba00113 \
+    --hash=sha256:eda689eda0c7301a80cf122dad28b1861e5605cbf455558f3775e1e8200e83a5
 snowballstemmer==2.0.0 \
     --hash=sha256:209f257d7533fdb3cb73bdbd24f436239ca3b2fa67d56f6ff88e86be08cc5ef0 \
     --hash=sha256:df3bac3df4c2c01363f3dd2cfa78cce2840a79b9f1c2d2de9ce8d31683992f52