[doc] add documentation about the XPath engine

- pylint searx/engines/xpath.py
- fix indentation of some long lines
- add logging
- add doc-strings

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

docs/dev/engine_overview.rst

@@ -43,7 +43,7 @@ argument                type        information
 categories              list        pages, in which the engine is working
 paging                  boolean     support multible pages
 time_range_support      boolean     support search time range
-engine_type             str         ``online`` by default, other possibles values are 
+engine_type             str         ``online`` by default, other possibles values are
                                     ``offline``, ``online_dictionnary``, ``online_currency``
 ======================= =========== ========================================================
 
@@ -100,6 +100,8 @@ example code
    paging = True
 
 
+.. _engine request:
+
 making a request
 ================
 
@@ -198,6 +200,8 @@ example code
        return params
 
 
+.. _engine results:
+
 returned results
 ================
 

docs/dev/index.rst

@@ -9,6 +9,7 @@ Developer documentation
    quickstart
    contribution_guide
    engine_overview
+   xpath_engine
    search_api
    plugins
    translation

docs/dev/xpath_engine.rst

@@ -0,0 +1,9 @@
+.. _xpath_engine:
+
+================
+The XPath engine
+================
+
+.. automodule:: searx.engines.xpath
+  :members:
+