Add stages and criteria for incubation process

[cornelius]

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[cornelius]

Notes from discussion today:

• Candidate projects are started from outside or inside OpenRail. Once they fulfill the minimum criteria for "Sandbox" they can be accepted.
• Some projects might never leave the "Sandbox" and move to "Retired" directly, such as Hackathon projects
• We might want to find better names than "Sandbox" and "Incubation" which are better understandable in our context
• The criteria for usage is too difficult at the stage "Incubation" to fulfill, it should be moved to a later stage
• Maybe add a stage "Early Adoption" which is mature and has some usage, but not as much as expected from a "Graduated" project yet.

[cornelius]

I will merge this PR now, so we have a clean state for addressing any further comments, and will open a new pull request which addresses the questions from #11 (comment) based on that.