tarsnap, tarsnap-keygen: add pages (#12219)

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>
Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>
Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>

pages/common/tarsnap-keygen.md

@@ -0,0 +1,12 @@
+# tarsnap-keygen
+
+> Generate a key file for use with Tarsnap, an online backup service.
+> More information: <https://www.tarsnap.com/man-tarsnap-keygen.1.html>.
+
+- Register a machine with the Tarsnap server:
+
+`sudo tarsnap-keygen --keyfile {{path/to/file.key}} --user {{user_email}} --machine {{machine_name}}`
+
+- Encrypt the key file (a passphrase will be requested twice):
+
+`sudo tarsnap-keygen --keyfile {{path/to/file.key}} --user {{user_email}} --machine {{machine_name}} --passphrased`