Remove v1 artifacts

Summary

There are still some artifacts of the v1 editor in v2:

• .gitignore has all the Node/JS stuff still in it
• beat/editor/js still exists
• doc/{index.rst,editor.rst,licenses.rst,img/,svg/} are all still from v1

What does it provide?

Remove all of the above:

• Remove all the JS stuff from .gitignore
• Remove the beat/editor/js directory
• Remove most of the the text in doc/index.rst and just have it point to doc/contribute.rst for now
• Remove doc/{editor.rst,licenses.rst}
• Save doc/{img/,svg/} as we'll probably want to adapt these later for v2 documentation