Cleanup instructions for Spegel, remove flag references.

[dereknola]

Some confusion over how to properly configure Spegel in rancher prompted a second pass over these docs.

• Remove references to flag configs (ie --flag). Thats not how 99% of people control RKE2.
• Move Metrics up the page and put it as a subsection just after "Enabling". Hopefully people will see this sooner and know that they want

embedded-registry: true
supervisor-metrics: true

Right from the start, instead of having them realize it later and need to restart all their clusters.