docs: define __DOXYGEN__ for doxygen builds
Some docs depended on __DOXYGEN__ but this was never defined anywhere.
@@ -241,6 +241,7 @@ MACRO_EXPANSION = YES
EXPAND_ONLY_PREDEF = YES
PREDEFINED = \
$(ENV_DOXYGEN_DEFINES) \
+ __DOXYGEN__=1 \
__attribute__(x)= \
_Static_assert()= \
IDF_DEPRECATED(X)= \
