Documented supported platforms with a support tier

[beorn7]

We would like to list the support of various platforms in the way it is done for etcd.

For example, the support for 32-bit architectures is currently a bit unclear. 386 on Linux is probably supported at tier 3 (using the etcd support tier definition). See also prometheus/prometheus#16690 to improve the support.

[Dharma-09]

We can definitely build something similar to Rust’s platform support tiers. While that page goes into a lot of detail. we can start with a simpler version that lists supported platforms along with tier definitions (e.g., Tier 1: fully supported and tested, Tier 3: best-effort).