Is there a way to back up agent jobs? I can't find documentation about whether agent jobs in SQL Server can be backed up. If not, how can I back them up in order to restore during the future recovery in case of an instance failure?
SQL agent jobs are stored in the system msdb database, so backing up that database will do what you need.

In a disaster, you can restore the entire msdb database, or, if necessary, recover individual jobs from a restored copy of msdb:

Serverfault: Recovering a specific job definition from an MSDB backup?

If you want them in a more immediately usable format, you can right-click and script out individual jobs manually, or if you'd like to script out all jobs on the server at once:

Stack Overflow: Automatically create scripts for all SQL Server Jobs