fix: #1631: add tox test support for documentation

Signed-off-by: Harsha Narayana <harsha2k4@gmail.com>

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36, py37, {py36,py37}-no-ext, lint, check, security
+envlist = py36, py37, {py36,py37}-no-ext, lint, check, security, docs
 
 [testenv]
 usedevelop = True
@@ -55,3 +55,14 @@ deps =
 
 commands =
     bandit --recursive sanic --skip B404,B101 --exclude sanic/reloader_helpers.py
+
+[testenv:docs]
+deps =
+    sphinx>=2.1.2
+    sphinx_rtd_theme>=0.4.3
+    recommonmark>=0.5.0
+    docutils
+    pygments
+
+commands =
+    make docs-test