Fix empty navigation tree in doxygen documentation output

[AniruddhaKanhere]

Description

The published API documentation at https://freertos.github.io/FreeRTOS-Plus-TCP/ renders pages but has an empty navigation tree — none of the documented files, types, or functions are reachable from the UI. The generated navtreedata.js contains only the (empty) main page:

var NAVTREE = [ [ "FreeRTOS-Plus-TCP", "index.html", ] ];

Root cause

docs/doxygen/layout.xml was authored for doxygen 1.8.20, where wrapping a visible <filelist> tab inside a <tab type="files" visible="no"> parent was the idiom to show a single flat "Files" entry. In doxygen 1.9.x and later, an invisible parent tab hides its entire subtree, so the file list never renders. Because classes/structs and globals are intentionally hidden in this layout, the file list is the only navigable content — hiding it leaves the navigation effectively empty.

Fix

Set the parent files tab to visible="yes". The file list renders again; globals and internal data structures remain hidden, preserving the original layout intent.

-    <tab type="files" visible="no" title="">
+    <tab type="files" visible="yes" title="">
       <tab type="filelist" visible="yes" title="Files" .../>
       <tab type="globals" visible="no" title="" intro=""/>
     </tab>

Test Steps

• Reproduced the empty navtree locally from the current config.doxyfile (doxygen 1.16.1) — matches the live site.
• After the change, a clean rebuild (doxygen docs/doxygen/config.doxyfile, exit 0, no new warnings) produces a populated navtree (Files → all 42 documented source files), and API functions such as FreeRTOS_socket are reachable.

Note

Already-deployed per-version sites on the gh-pages branch will remain broken until regenerated and redeployed; this change fixes future doxygen builds.