Chinese Simplified (zh-CN): Catch-up with EN version

[henrif75]

Translate remaining messages and review fuzzy translations.
As of 6/5/24, current translation stats are:

zh-CN.po
2619 translated messages, 731 fuzzy translations, 423 untranslated messages.

Steps:

1. Sync your branch with main.
2. Recreate messages.pot:

MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot", "granularity": 0}}' \
  mdbook build -d po

3. Merge with the zh-CN.po file:

msgmerge --update po/zh-CN.po po/messages.pot

4. Edit the zh-CN.po file with your favorite PO editor.

[mgeisler]

The live translation delta can be seen in the synced translation report.