Add mdbook-xgettext skip for untranslatable codeblocks in Bare Metal

[0scvr]

See #1327

Removes 280 lines from messages.pot.

[0scvr]

Can still be improved, but I think this is good enough for now.

[mgeisler]

Nice, this removed 280 lines from the translations:

% diff po/messages-{before.pot,after.pot} | diffstat
 messages-after.pot |  286 -----------------------------------------------------------------------
 1 file changed, 3 insertions(+), 283 deletions(-)