Bump jinja2 from 2.8 to 2.11.3 in /doc

Bumps [jinja2](https://github.com/pallets/jinja) from 2.8 to 2.11.3.
- [Release notes](https://github.com/pallets/jinja/releases)
- [Changelog](https://github.com/pallets/jinja/blob/master/CHANGES.rst)
- [Commits](https://github.com/pallets/jinja/compare/2.8...2.11.3)

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

doc/requirements.txt

@@ -6,7 +6,7 @@ backports.ssl-match-hostname==3.5.0.1
 certifi==2015.11.20.1
 CommonMark==0.5.4
 docutils==0.12
-Jinja2==2.8
+Jinja2==2.11.3
 livereload==2.4.1
 MarkupSafe==0.23
 pathtools==0.1.2