Introduce the community roadmap

[oleg-nenashev]

It would be nice to create a GitHub project that provides a non-technical community roadmap

Acceptance criteria

• Key community issues are created in the repositories
• There is a GitHub project that allows navigating the roadmap: https://github.com/orgs/keptn/projects/1
• Community roadmap is referenced in ROADMAP.md files where relevant
• Community roadmap is reviewed at Keptn community meetings and signed off there
• There is a public announcement

[oleg-nenashev]

https://keptn.sh/docs/roadmap/ also needs to be updated