fix(docz-theme-default): mobile menu following anchor clicks (#713)

whenever a user clicked an anchor link with the mobile menu open it
would cause the mobile menu to re-position itself to half-way off the
page

core/docz-theme-default/src/components/shared/Sidebar/index.tsx

@@ -45,6 +45,7 @@ const Wrapper = styled.div<WrapperProps>`
 
   @media screen and (max-width: ${breakpoints.desktop - 1}px) {
     transform: translateX(${p => (p.opened ? '-100%' : '0')});
+    position: ${p => (p.opened ? 'auto' : 'fixed')};
   }
 
   ${get('styles.sidebar')};