docs: black screen after idle is a failed s2idle resume

[Weijtmans]

Adds the one intermittent-black-screen case the existing list does not cover: the box is not overheating or crashing, it suspended and never came back. The BC-250 offers only s2idle and resume does not work on this hardware, so an idle timeout turns into a hang that needs a power cut.

What the new subsection gives:

• The distinguishing symptoms: no input wakes it and the network is dead too, because nothing is running anymore.
• The one-command diagnosis: the previous boot's journal ends at PM: suspend entry (s2idle) with no resume line (journalctl -b -1 -n 5).
• The fix in two layers: the desktop's idle-suspend setting, plus masking the four suspend targets system-wide, because Steam Game Mode has its own idle suspend that desktop settings do not reach. Revert command included.

Also adds a matching entry to the Bazzite page's Known Issues that links here, since that is where an affected user is likely to look first.

First-party: hit, diagnosed and fixed on the tested board.

mkdocs build --strict passes.