nixos-rebuild: add example for list-generations command

[bittner]

Adds an example for the list-generations subcommand, which is not mentioned in the online docs but in the manpage.

Fixes #16307

• The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• The page(s) have at most 8 examples.
• The page description(s) have links to documentation or a homepage.
• The page(s) follow the content guidelines.
• The page(s) follow the style guide.
• The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[tldr-bot]

Hello! I've noticed something unusual when checking this PR:

• The page pages.de/linux/nixos-rebuild.md is outdated, based on the command contents itself, compared to the English page.
• The page pages.ko/linux/nixos-rebuild.md is outdated, based on the command contents itself, compared to the English page.

Is this intended? If so, just ignore this comment. Otherwise, please double-check the commits.

[bittner]

@Managor Applied the fixes and rebased.

Sorry for the force-push, that was GitHub, not me. I probably should have let it add a merge commit instead of rebase + force push. 🤔