Consildate several pages under Add-Ons section

[dereknola]

Changes

Moved helm, Import Images, and Storage pages under an Add-Ons section, which encompasses additional functionality that is bundled into K3s, but comes from other software packages (helm, spegel, longhorn etc..).

Future Work

Move information about GPUs and Alternative Runtimes under this section.

[brandond]

This is a little odd since "AddOn" is the CRD type that we use to track deployed manifests in the manifests dir: see kubectl get addon -A and https://github.com/k3s-io/api/blob/v0.1.3/k3s.cattle.io/v1/types.go#L14-L23. Harvester and Rancher also use the same term for Kubernetes manifests that are automatically deployed to the cluster, and RKE1 did the same (which is why it got called an AddOn here in K3s).

I think we should call this something else, like "packaged components" although that is also a mouthful.

Some of our packaged components are deployed as Add-Ons, some are built into the binary. User-provided auto-deploying manifests would also be considered Add-Ons.

[dereknola]

We went with Add-Ons for the RKE2 docs as well, so I was matching naming. Packaged Components makes me think of the stuff like crictl, busybox, and all the other binaries that K3s is shipped with, not the Feature Functionality of it.

Integrations is not the right word because the other software producers aren't working "with K3s" in some way, we are co-opting the software.

I don't have a good word for "A feature of K3s that exists because of external software packages, but require user intervention to operate (e.g. place manifests, deploy PVCs, place image.tar.gz files)"

How about "Extensions"?