Added DOC_EXCLUDE_SECTION section tag to conditionally remove sections
from FreeRTOS source files which we don't want in IDF API docs.
Added DOC_SINGLE_GROUP section tag to enable a flat document for
FreeRTOS and removing groups.
With these conditional tags, we can upstream FreeRTOS files without
affecting the upstream documentation.
Signed-off-by: Sudeep Mohanty <sudeep.mohanty@espressif.com>