English Documentation added for the latest version

<https://ethna-english-documentation.readthedocs.org/en/latest/>
English Doc is written almost 70%.  First, will be looking forward to your the feedback including missing articles etc..

On the original doc for ``ethna.jp`` Made one change to the ``index.rst`` i.e. added a rel link to the ``en/_build/html/index/html`` as ``.. seealso::``.

Cheers, Thanks