[doc] engine tables: show engines in all categories

Previously the documentation grouped the engines by their first
category so e.g. YouTube and Invidious were only shown in the
in the videos section but not in the music section.

This commit fixes this by iterating over searx.engines.categories,
which also has the added benefit that the sections are now in the
same order as the tabs in the user interface.

docs/conf.py

@@ -40,6 +40,7 @@ exclude_patterns = ['build-templates/*.rst']
 import searx.engines
 import searx.plugins
 searx.engines.load_engines(searx.settings['engines'])
+
 jinja_contexts = {
     'searx': {
         'engines': searx.engines.engines,
@@ -48,13 +49,14 @@ jinja_contexts = {
             'node': os.getenv('NODE_MINIMUM_VERSION')
         },
         'enabled_engine_count': sum(not x.disabled for x in searx.engines.engines.values()),
+        'categories': searx.engines.categories,
     },
 }
 jinja_filters = {
     'sort_engines':
     lambda engines: sorted(
         engines,
-        key=lambda engine: (engine[1].about.get('language', ''), engine[0])
+        key=lambda engine: (engine.about.get('language', ''), engine.name)
     )
 }