Bump jinja2 from 2.11.3 to 3.1.3 in /docs

Bumps [jinja2](https://github.com/pallets/jinja) from 2.11.3 to 3.1.3.
- [Release notes](https://github.com/pallets/jinja/releases)
- [Changelog](https://github.com/pallets/jinja/blob/main/CHANGES.rst)
- [Commits](pallets/jinja@2.11.3...3.1.3)

---
updated-dependencies:
- dependency-name: jinja2
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -5,7 +5,7 @@ chardet==3.0.4
 docutils==0.14
 idna==2.7
 imagesize==0.7.1
-Jinja2==2.11.3
+Jinja2==3.1.3
 MarkupSafe==0.23
 Pygments==2.15.0
 pytz==2018.7