doc: nrf: add nrf_security APIs to the Doxygen input

So that they are processed and rendered by Doxygen. Note that grouping
is still needed so that headers appear nicely in the "Topics" page.
This is a task left to the maintainers of such code.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/nrf/nrf.doxyfile.in

@@ -958,7 +958,8 @@ WARN_LOGFILE           =
 
 INPUT                  = @DOCSET_SOURCE_BASE@/applications \
                          @DOCSET_SOURCE_BASE@/lib \
-                         @DOCSET_SOURCE_BASE@/include
+                         @DOCSET_SOURCE_BASE@/include \
+                         @DOCSET_SOURCE_BASE@/subsys/nrf_security/include
 
 # This tag can be used to specify the character encoding of the source files
 # that Doxygen parses. Internally Doxygen uses the UTF-8 encoding. Doxygen uses