feat: keep a replaced BIOS file beside its folder

A file in place whose contents differ (another revision, a personal
dump) was overwritten and lost. It is now moved to
retrobios-backup/<date-time>/ beside the folder written to, under the
same path, before the verified copy replaces it. Beside, not inside:
DuckStation, SwanStation and Rustation read every file of the BIOS
folder and BizHawk walks its firmware folder recursively.

Identical files are never kept. --dry-run counts what would be kept,
the summary and the summary event name the folder, files and bytes,
and --restore FOLDER puts the files back. Data trees and standalone
copies keep their replaced files the same way.

scripts/generate_site.py

@@ -498,7 +498,8 @@ def generate_home(
         "## Is this safe?",
         "",
         "The installer writes only inside your BIOS folder, checks every file "
-        "before saving it, and never touches games or saves. "
+        "before saving it, keeps any file of yours it replaces in a "
+        "retrobios-backup folder beside it, and never touches games or saves. "
         "[How the command is checked](wiki/faq.md#is-piping-the-installer-into-a-shell-safe).",
         "",
         "Laws on copying console firmware differ by country. "