[mod] documentation: change the jinja context doesn't depend on searx.webapp

Before this commit, in the documentation, the jinja context is 'webapp' and contains
the global variable in the searx.webapp module.

This commit changes this to include only the mandatory variables to build the
documentation.

docs/conf.py

@@ -27,9 +27,15 @@ numfig = True
 
 exclude_patterns = ['build-templates/*.rst']
 
-from searx import webapp
+import searx.search
+import searx.engines
+import searx.plugins
+searx.search.initialize()
 jinja_contexts = {
-    'webapp': dict(**webapp.__dict__),
+    'searx': {
+        'engines': searx.engines.engines,
+        'plugins': searx.plugins.plugins
+    },
 }
 
 # usage::   lorem :patch:`f373169` ipsum