Add Rust custom environment

[hariby]

Issue #, if available:

Description of changes:

Added an example of Rust custom environment under studio-lab-examples/custom-environments/Rust. This example consists of:

• rust.yml: base yml to create an empty environment named rust,
• install.sh: body of the install script to create the Rust Jupyter kernel,
• uninstall.sh: to uninstall Rust kernel.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[hariby]

After reading comments on another PR #50, I am considering to

• clear the Jupyter output
• add DS/ML-related content