common curriculum: separate out

[reshamas]

What is end state:

• copy out [common-theme] from main curriculum
• common docs: should it stay in main curriculum or keep with common theme
• we need a first draft of a release plan for the theme specified in the GitHub folder (in new repo .github folder)

Tickets

• [ ] #1178
• [ ] #1177
• [ ] #1179

[SallyMcGrath]

https://github.com/CodeYourFuture/common-theme