Make Mathics Binder-ready and add Binder button

[slel]

This is to make it easy to use Mathics via Binder.

• prepare the Mathics repo for Binder
• add "Launch Binder" button on the Mathics repo's readme
• add information on Mathics home page about running Mathics in Binder

See this comment by @wolfv in an earlier issue:

• Documentation points to mathics.net which is hopelessly out-of-date #762 (comment)

In fact @wolfv's fork of the Mathics repo is Binder-ready, see:

• https://github.com/wolfv/Mathics/tree/binder

[slel]

See also

• discussion at Documentation points to mathics.net which is hopelessly out-of-date #762 (comment)
• Make Mathics work on CoCalc #916 : make Mathics work on CoCalc