Add Bottlerocket information to docs

**What would you like to be added**:
There's no reference to Bottlerocket in EKS-A installation documentation and no cross reference to more detailed Bottlerocket docs on the Artifacts page.

**Why is this needed**:
In the installation docs, customers aren't told that Bottlerocket is available and on the artifacts docs they don't know how to learn more about Bottlerocket.