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

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+# 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`

pages/common/tarsnap.md

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+# tarsnap
+
+> Manipulate remote Tarsnap encrypted backups.
+> Note: you don't need to specify the key file and the cache directory if you configure them in `/usr/local/etc/tarsnap.conf` or `~/.tarsnaprc`.
+> See also: `tarsnap-keygen`.
+> More information: <https://www.tarsnap.com/man-tarsnap.1.html>.
+
+- [c]reate a backup archive of one or more files or directories, specifying the cryptographic key and the cache directory:
+
+`tarsnap -c --keyfile {{path/to/key_file}} --cachedir {{path/to/cache_directory}} -f {{archive_name}} {{path/to/file_or_directory1 path/to/file_or_directory2 ...}}`
+
+- Display how much data would be uploaded:
+
+`tarsnap -c --dry-run --print-stats --keyfile {{path/to/key_file}} --cachedir {{path/to/cache_directory}} -f {{archive_name}} {{path/to/file_or_directory1 path/to/file_or_directory2 ...}}`
+
+- List stored archives:
+
+`tarsnap --list-archives --keyfile {{path/to/key_file}}`
+
+- [d]elete a specific archive:
+
+`tarsnap -d --keyfile {{path/to/key_file}} --cachedir {{path/to/cache_directory}} -f {{archive_name}}`
+
+- Lis[t] the contents of a specific archive in [v]erbose mode:
+
+`tarsnap -tv --keyfile {{path/to/key_file}} -f {{archive_name}}`
+
+- Restore one or more files or directories from a specific archive:
+
+`tarsnap -x --keyfile {{path/to/key_file}} -f {{archive_name}} {{path/to/file_or_directory1 path/to/file_or_directory2 ...}}`
+
+- Copy an archive:
+
+`tarsnap -c --keyfile {{path/to/key_file}} -f {{new_archive_name}} @@{{source_archive_name}}`