doc: support sphinx 3.0.0+

It seems the :function: directive is much stricter now and requires valid C declaration. The lua functions seem to work okay with :py-function:. Some of the doxygen generated content was invalid and required various hacks. Now it's working, but I had to omit some symbols.

This should hopefully fix readthedocs issue we're experiencing right now (https://github.com/michaeljones/breathe/issues/495) by switching to sphinx 3.0.0+ in readhthedocs.

doc/Doxyfile

@@ -17,4 +17,7 @@ HIDE_UNDOC_MEMBERS = YES
 HIDE_UNDOC_CLASSES = YES
 OPTIMIZE_OUTPUT_FOR_C = YES
 ENABLE_PREPROCESSING  = YES
-PREDEFINED        = NDEBUG
+MACRO_EXPANSION   = YES
+EXPAND_ONLY_PREDEF     = YES
+PREDEFINED        = NDEBUG KR_CONST= KR_EXPORT= KR_PURE= lru_t(type)=see_source_code KR_PRINTF(n)= KR_COLD= array_t(type)=see_source_code
+EXCLUDE_SYMBOLS = static_assert uint set_walk_cb module_api_cb kr_prop_cb kr_straddr_split