Upgrade rtd setting

[disktnk]

• apply v2 format
• use conda env
  • to build chainerx modules
  • currently, chainer force to build chainerx when rtd env
  • https://github.com/chainer/chainer/blob/54966403f2d4ceca91c041adc272b7372d773dd1/setup.py#L188-L190
• use extras_require to install chainer module
  • currenctly chainerui does not have to install chainer to install, but docs references chainer module
  • unable to use python.install.requriements = docs/requirements.txt, I don't no the reason
  • https://docs.readthedocs.io/en/latest/config-file/v2.html#requirements-file

fixes #283