docs: fix doc/index.rst for moved IGA section, reference tutorial
docs: fix doc/index.rst for moved IGA section, reference tutorial

rc/sfepy,rc/sfepy,RexFuzzle/sfepy,lokik/sfepy,RexFuzzle/sfepy,vlukes/sfepy,lokik/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,vlukes/sfepy,lokik/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy,lokik/sfepy,sfepy/sfepy,BubuLK/sfepy,sfepy/sfepy,sfepy/sfepy