doc: Update cockpit/ws documentation
doc: Update cockpit/ws documentation

* Project Atomic was superseded by Fedora CoreOS.
 * It's not a "docker" container, it's just a container.
 * Speaking about "the default" is misleading, as the container isn't
   installed by default. But we only support the container there, not
   trying to squeeze the web server into an OSTree overlay.

Closes #13153
moraleslazaro/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,garrett/cockpit,deryni/cockpit,mvollmer/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpituous/cockpit,cockpit-project/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpituous/cockpit