Docs: Move magic-number from process to staging

'Other material' section on 'process/index' is for unsorted documents.
However we also have a dedicated place for the purpose, 'staging/'.
Move 'magic-number' from the section to 'staging/' directory.

Signed-off-by: SeongJae Park <sj@kernel.org>
Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240624185312.94537-4-sj@kernel.org