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

@@ -9,7 +9,7 @@ docutils==0.15.2
 exhale==0.2.3
 idna==2.8
 imagesize==1.1.0
-Jinja2==2.11.3
+Jinja2==3.1.3
 lxml==4.9.1
 MarkupSafe==1.1.1
 packaging==19.2