Docs update to better integrate overlays info

Describe the issue

Right now our kubernetes install documentation (https://docs.sourcegraph.com/admin/install/kubernetes#steps) has a couple of things that have confused users:

1. The steps are an unordered list, which seems confusing since they are sequential (but maybe there's context I'm missing)
2. Right now, we don't link out to the overlays documentation or point out to users when what's in the install docs will diverge for those using overlays: "Deploy the desired version of Sourcegraph to your cluster: ./kubectl-apply-all.sh".

Where is the issue located?

https://docs.sourcegraph.com/admin/install/kubernetes#steps

Suggested resolution

Per the conversation at https://sourcegraph.slack.com/archives/CJX299FGE/p1616607364105000, I feel like at that step we should either link out to the overlay documentation (https://docs.sourcegraph.com/admin/install/kubernetes/configure#overlay-basic-principles) and clarify that that process stands in for the apply-all script.