[docs] add new rubric 'Source-Code'

- document namespace: searx.engines
- move docs/dev/xpath_engine.rst -> docs/src/searx.engines.xpath.rst

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

docs/src/index.rst

@@ -0,0 +1,14 @@
+===========
+Source-Code
+===========
+
+This is a partial documentation of our source code.  We are not aim to document
+every item from the source code, but we will add documentation when requested.
+
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents
+   :glob:
+
+   searx.*