[Docs] Mark DASHBOARD_SETUP_MODE deprecated in the runtime-configuration table (#4322)

The feature-controls table presented DASHBOARD_SETUP_MODE as an active
control that enables first-run setup, while the setup-mode section of
the same file states the variable is deprecated and that setup.mode in
the router config is the only source. An operator who configures the
setup flow from the table gets no setup flow: the variable is still
read, but only so a disagreement with the config file is reported on
/api/setup/state and in the backend log, never to enable setup mode.

The row now marks the variable deprecated, keeps the disagreement-signal
scope, and points to setup.mode as the control, matching the section
below and the backend's own comments. The env-var reader, the
/api/setup/state behavior, and any startup hard-fail stay out of scope
per the maintainer discussion on the issue.

Signed-off-by: Zi-Yi-Ming <202411103024@stu.sdjzu.edu.cn>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>