Clarify init container, regular init container, non-init container and sidecar container

**This is a Feature Request**

The use of init container, non-init container, regular init container and sidecar container is fairly confusing in this doc https://github.com/kubernetes/website/blob/main/content/en/docs/concepts/workloads/pods/sidecar-containers.md#resource-sharing-within-containers . For example in the resource-sharing-within-containers section it is not clean if (non-init containers) excludes init containers with `restartPolicy` set to `Always` or only excludes `regular init containers` as defined earlier in the doc.

**What would you like to be added**

This PR attempts to make that clarification https://github.com/kubernetes/website/pull/52509

**Why is this needed**

The terminology for sidecar containers is fairly confusing. In the pointed out section, the problem is especially acute.

**Comments**