docs: shuffle files to have the same logical hierarchy as on the generated doc

Also:
 - rename some files to remove redundant information from the filenames
 - rename many labels to match the filename and its place in the hierarchy
 - move lua_api under the scripting section

docs/rst/scripting/lua_api/lua_session_item_api.rst

@@ -0,0 +1,42 @@
+ .. _lua_session_item_api:
+
+Session Item
+============
+
+Lua objects that bind a :ref:`WpSessionItem <session_item_api>`
+contain the following methods:
+
+.. function:: SessionItem.get_associated_proxy(self, type)
+
+   Binds :c:func:`wp_session_item_get_associated_proxy`
+
+   :param self: the session item
+   :param string type: the proxy type name
+   :returns: the proxy object or nil
+
+.. function:: SessionItem.reset(self)
+
+   Binds :c:func:`wp_session_item_reset`
+
+   :param self: the session item
+
+.. function:: SessionItem.configure(self, properties)
+
+   Binds :c:func:`wp_session_item_configure`
+
+   :param self: the session item
+   :param table properties: The configuration properties
+   :returns: true on success, false on failure
+   :rtype: boolean
+
+.. function:: SessionItem.register(self)
+
+   Binds :c:func:`wp_session_item_register`
+
+   :param self: the session item
+
+.. function:: SessionItem.remove(self)
+
+   Binds :c:func:`wp_session_item_remove`
+
+   :param self: the session item