monitoring(frontend): downgrade page_load_latency to warning, update next steps

Created by: bobheadxi

There seems to be some evidence that this is not really a critical issue: https://github.com/sourcegraph/sourcegraph/issues/36434#issuecomment-1144458258 For now, we should just downgrade this to a warning alert.

I've also moved the Interpretation back into the NextSteps here.

Test plan

n/a