This builds on the work of @dyoo in
https://github.com/google/mdbook-i18n-helpers/pull/69: by adding a
special `<!-- mdbook-xgettext: skip -->` comment, we can skip the
following code block.
I also modified a few code blocks to remove translatable text: variable
names are not expected to be translated, so it’s fine to have a line
with `println!("foo: {foo}")` in the code block.
This PR removes 36 messages from the POT file. The number of lines drop
by 633 (3%).
Part of #1257.
d0e0e5c1af