Doc: List all Qt libraries the xcb platform plugin needs on Linux

The Linux deployment guide lists the files needed to ship the Plug &
Paint example, but leaves out two Qt libraries the xcb platform plugin
needs at runtime: libQt6XcbQpa, which the plugin links directly, and
libQt6DBus, which Qt GUI links when Qt is built with D-Bus support, the
default on Linux. An application deployed by following the table fails
to load the platform plugin, or silently picks up an incompatible copy
of the library from elsewhere on the system.

The guide also suggests running ldd on the application to find its
dependencies. That misses everything plugins depend on, because Qt
loads them at runtime.

Add both libraries to the table, and advise running ldd on each
deployed plugin as well.

Fixes: QTBUG-46340
Pick-to: 6.12
Change-Id: Ib8aa5b633c86a224bd1fd11ee746e0a64d14b757
Reviewed-by: David Boddie <david.boddie@qt.io>