docs: Pin jinja2 to < 3.1.0

Jinja2 3.1.0 removed deprecated code that is still used by sphinx
1.8.x, and it looks like our custom sphinx extensions are not working
with more recent versions of sphinx..

See:
- pallets/jinja#1631
- readthedocs/readthedocs.org#9037

and

- PowerDNS#7712

The exact error is:
```
Extension error:
Could not import extension sphinx.builders.latex (exception: cannot import name 'contextfunction' from 'jinja2' (/dnsdist/pdns/dnsdistdist/.venv/lib/python3.7/site-packages/jinja2/__init__.py))
```

(cherry picked from commit 92ad297)

docs/requirements.txt

@@ -4,4 +4,5 @@ git+https://github.com/pieterlexis/sphinx-jsondomain@no-type-links
 git+https://github.com/pieterlexis/sphinx-changelog@render-tags
 sphinxcontrib-fulltoc
 guzzle_sphinx_theme
-docutils!=0.15
+docutils!=0.15,<0.18
+jinja2<3.1.0

pdns/dnsdistdist/docs/requirements.txt

@@ -4,4 +4,5 @@ git+https://github.com/pieterlexis/sphinx-jsondomain@no-type-links
 git+https://github.com/pieterlexis/sphinx-changelog@render-tags
 sphinxcontrib-httpdomain
 sphinxcontrib-fulltoc
-docutils!=0.15
+docutils!=0.15,<0.18
+jinja2<3.1.0

pdns/recursordist/docs/requirements.txt

@@ -5,4 +5,5 @@ git+https://github.com/pieterlexis/sphinx-changelog@render-tags
 guzzle_sphinx_theme
 sphinxcontrib.httpdomain
 sphinxcontrib-fulltoc
-docutils!=0.15
+docutils!=0.15,<0.18
+jinja2<3.1.0