`setup.md` is the page someone follows on a fresh deploy, and it had no
step for authelia at all — so the operator finished the walkthrough with
a crash-looping container and a 502, having done everything the doc
asked. The warnings added to sso.md and gateway.md only help someone who
is already debugging; this is the page that decides whether they end up
debugging.
New step 3, gated on `swarm.authelia.enable`, placed before matrix
because it is part of getting the front door open. Says plainly that it
is required to finish the install, names the symptom it prevents, and
links sso.md for the detail rather than restating it.
Renumbers matrix (4), spawn (5) and host commands (6), including the
matrix block's own 3a–3e labels, which would otherwise disagree with
their heading.
