[FEATURE] Handle Global Menu depth independently from toc

If the toctree on the root page is limited to a certain level count, like `:maxdepth: 1` still the complete page tree is displayed in Sphinx.

However as not ever project needs a full document tree and the creation of the document tree is time-consuming, an option for the guides.xml, max-menu-depth, was introduced. `max-menu-depth = 0` disables the global menu all together.

Author: linawolf

resources/schema/guides.xsd

@@ -28,6 +28,7 @@
         <xsd:attribute name="theme" type="xsd:string"/>
         <xsd:attribute name="default-code-language" type="xsd:string"/>
         <xsd:attribute name="links-are-relative" type="xsd:string"/>
+        <xsd:attribute name="max-menu-depth" type="xsd:int"/>
 
     </xsd:complexType>