Revert back to Dockerfile

[kwankyu]

Fixes #15, and prepare for sagemath/sage#36144.

This PR refreshes the repo in the master branch from ground up, by pulling in https://github.com/kwankyu/sage-binder-env

• We go back to Dockerfile to create the Binder environment.
• Removed mostly out-dated or obsolete files and contents.
• Updated the notebook "Start here" kept in notebooks directory
• We use JupyterLab instead of classical Jupyter notebook.

The base sagemath image used is the develop version created by https://github.com/sagemath/sage/blob/develop/.github/workflows/push_to_docker_hub.yml Hence it is the most recent available.

[kwankyu]

@dimpase, may I get maintainer role for this repo?

I want to act quickly when there is a problem with it, or when there is an improvement.

[dimpase]

@kwankyu - done.

[kwankyu]

Thanks!