docs: the quickstart deploys to staging, which is down #6279

Problem

The docs deploy a first realm with -chainid staging to rpc.staging.gno.land, which now returns 503, and faucet.gno.land does not serve staging anyway. A new builder following the docs fails at their very first deploy. This applies to docs/builders/quickstart.md, getting-started.md, tutorial-minisocial.md and contributor-guide.md, and to the same pages on docs.gno.land.

#6205 and #6217 moved these pages to onyx but were closed unmerged, so the reason they were closed needs to be settled first.

To do

• Agree on the target network for the docs (onyx?) and on why docs: flag the broken installer path and move the quickstart deploy off halted staging #6205 and docs: move the deploy walkthroughs off Staging, which stopped producing blocks on 2026-07-10 #6217 were closed.
• Check that a fresh deploy on onyx becomes callable without manual steps (deploys can "park" on onyx).
• Repoint every staging reference in docs/, faucet step included.

Done when

Someone with no context can follow the quickstart from install to a called realm.

Comes from #5459, #5460, #5462 and #5465.