Most of the Debian software RAID guides are out of date in one way or another I'm afraid. Almost all of them talk about the Debian installer not being able to install a working bootable RAID1 system (with / on the RAID1 array).

In fact the current installer (even in the stable version of Debian) is perfectly capable of achieving a bootable RAID1 system.
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Remind me to change my signature to something more interesting someday