Add RunCode Cloud instructions

[amakarudze]

This PR introduces cloud development instructions using RunCode.io so attendees don't have to go through the long installation process, but instead can code in the cloud.

Changes in this pull request:

• Add Installation (RunCode setup) link to SUMMARY.md.
• Add cloud_development_setup folder with README.md and instructions.md for instructions on setting up RunCode Cloud environment.
• Add RunCode section to intro_to_command_line/open_instructions.md.
• Add RunCode section to installation/README.md.
• Add RunCode Django project setup instructions to django_start_project/README.md
• Add Runcode instructions on opening Python prompt to python_introduction/prompt.md

[das-g]

I'll take a closer look at this PR later this week.

Meanwhile, I'd like to suggest to use "semantic linefeeds" a.k.a. "semantic line breaks" for newly added paragraphs. Though most of the Django Girls tutorial isn't written that way, placing source line breaks at semantically meaningful spots it the text (and especially between sentences) facilitates both, commenting on individual statements or thoughts in the text as part of the review process, as well as future editing and diff-ing.