build(deps): bump docutils from 0.18.1 to 0.20.1 in /lib/spack/docs

Bumps [docutils](https://docutils.sourceforge.io/) from 0.18.1 to 0.20.1.

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

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

Author: dependabot[bot]

lib/spack/docs/requirements.txt

@@ -3,6 +3,6 @@ sphinxcontrib-programoutput==0.17
 sphinx_design==0.4.1
 sphinx-rtd-theme==1.2.2
 python-levenshtein==0.21.0
-docutils==0.18.1
+docutils==0.20.1
 pygments==2.15.1
 urllib3==2.0.3