Skip some untranslatable code blocks (#1258)

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.

src/memory-management/scope-based.md

@@ -24,6 +24,7 @@ void say_hello(std::unique_ptr<Person> person) {
 
 Special move constructors are used when passing ownership to a function:
 
+<!-- mdbook-xgettext: skip -->
 ```c++
 std::unique_ptr<Person> person = find_person("Carla");
 say_hello(std::move(person));