[doc/index] when creating TOC, go max 3 levels down into sub-chapters
[doc/index] when creating TOC, go max 3 levels down into sub-chapters

Signed-off-by: Robert Wuttke <12e9293ec6b30c7fa8a0926af42807e929c1684f@benocs.com>

Benocs/core,Benocs/core,Benocs/core,Benocs/core,Benocs/core