Release cycle information and expectation

[dholbach]

We have https://github.com/fluxcd/flux2/blob/main/docs/internal/release.md but we currently are lacking user-targetted

security-relevant information such as how long versions are supported and how often to expect releases.
Examples of open source projects that describe their release cycle:
● Kubernetes: https://kubernetes.io/releases/version-skew-policy/
● Redis: https://redis.io/topics/releases
● Linux kernel: https://www.kernel.org/category/releases.html

Maybe make this a section in a top-level /docs/security page?

[dholbach]

#598 is a meta tracking issue.

[pjbgf]

We already have some of this information in the Migration Timetable page (e.g. support / version skew policy).

But things such as release cycle, version support and skew policy deserve a standalone page. Specially when considering that users new to flux that are outside the "migration path" will not pay much attention to the migration timetable.