Korean (ko): Catch-up with EN version

Translate remaining messages and review fuzzy translations.
As of 6/5/24, current translation stats are:
```
ko.po
2646 translated messages, 619 fuzzy translations, 508 untranslated messages.
```
### Steps:
1. Sync your branch with `main`.
2. Recreate `messages.pot`:
```shell
MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot", "granularity": 0}}' \
mdbook build -d po
```
3. Merge with the `ko.po`:
```shell
msgmerge --update po/ko.po po/messages.pot
```
4. Edit the `ko.po` file with your favorite PO editor.