Restore "API Documentation" in the main menu

Between Doxygen 1.8.17 and Doxygen 1.9.8, the top-level "modules" tab
type in the Doxygen layout has changed such that our "API Documentation"
section is no longer generated and is instead considered of the type
"topics". Update our layout XML to reflect this change and restore this
critical section of the documentation.

Fixes #1563.

docs/DoxygenLayout.xml

@@ -3,7 +3,8 @@
   <!-- Navigation index tabs for HTML output -->
   <navindex>
     <tab type="mainpage" visible="yes" title="Introduction"></tab>
-    <tab type="modules" visible="yes" title="API Documentation" intro="These are the libraries supplied in the Raspberry Pi Pico SDK"/>
+    <tab type="topics" visible="yes" title="API Documentation" intro="These are the libraries supplied in the Raspberry Pi Pico SDK"/>
+    <tab type="modules" visible="no" title="" intro=""/>
     <tab type="user" url="@ref examples_page" visible="yes" title="Examples" intro="Links to SDK examples"/>
     <tab type="usergroup" url="@ref weblinks_page" visible="yes" title="Additional Documentation" intro="Links to datasheets and documentation">
       <tab type="user" url="https://rptl.io/pico-datasheet" visible="yes" title="Raspberry Pi Pico Datasheet" intro=""/>