Bump docutils from 0.16 to 0.18 in /docs

Bumps [docutils](http://docutils.sourceforge.net/) from 0.16 to 0.18.

---
updated-dependencies:
- dependency-name: docutils
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

docs/requirements.txt

@@ -1,5 +1,5 @@
 sphinx==4.2.0
-docutils==0.16
+docutils==0.18
 Pygments==2.10.0
 sphinx-rtd-theme==1.0.0
 sphinxcontrib-websupport==1.2.4