[mod] replace /help by /info pages and include pages in project docs

This patch implements a bolierplate to share content from info-pages of the
SearXNG instance (URL /info) with the project documentation (path /docs/user).

The info pages are using Markdown (CommonMark), to include them in the project
documentation (reST) the myst-parser [1] is used in the Sphinx-doc build chain.

If base_url is known (defined in settings.yml) links to the instance are also
inserted into the project documentation::

    searxng_extra/docs_prebuild

[1] https://www.sphinx-doc.org/en/master/usage/markdown.html

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

tests/unit/test_webapp.py

@@ -177,10 +177,14 @@ class ViewsTestCase(SearxTestCase):
 
         self.assertIn(b'<description>first test content</description>', result.data)
 
-    def test_about(self):
-        result = self.app.get('/help/en/about')
+    def test_redirect_about(self):
+        result = self.app.get('/about')
+        self.assertEqual(result.status_code, 302)
+
+    def test_info_page(self):
+        result = self.app.get('/info/en/search-syntax')
         self.assertEqual(result.status_code, 200)
-        self.assertIn(b'<h1>About SearXNG</h1>', result.data)
+        self.assertIn(b'<h1>Search syntax</h1>', result.data)
 
     def test_health(self):
         result = self.app.get('/healthz')