add docs

docs/Makefile

docs/make.bat

docs/source/conf.py

docs/source/convlab2.dialog_agent.rst

docs/source/convlab2.dst.comer.multiwoz.models.rst

docs/source/convlab2.dst.comer.multiwoz.rst

docs/source/convlab2.dst.comer.rst

docs/source/convlab2.dst.mdbt.multiwoz.rst

docs/source/convlab2.dst.mdbt.rst

docs/source/convlab2.dst.rst

docs/source/convlab2.dst.rule.camrest.rst

docs/source/convlab2.dst.rule.crosswoz.rst

docs/source/convlab2.dst.rule.multiwoz.rst

docs/source/convlab2.dst.rule.rst

docs/source/convlab2.dst.sumbt.multiwoz.rst

docs/source/convlab2.dst.sumbt.rst

docs/source/convlab2.dst.trade.multiwoz.rst

docs/source/convlab2.dst.trade.rst

docs/source/convlab2.e2e.damd.multiwoz.rst

docs/source/convlab2.e2e.damd.rst