delete old docs (#327)

* delete old docs

* new docs

docs/pages/cli/backup.md

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+# Backup
+
+```bash
+autorestic backup [-l, --location] [-a, --all]
+```
+
+Performs a backup of all locations if the `-a` flag is passed. To only backup some locations pass one or more `-l` or `--location` flags.
+
+```bash
+# All
+autorestic backup -a
+
+# Some
+autorestic backup -l foo -l bar
+```
+
+## Specific location
+
+`autorestic` also allows selecting specific backends for a location with the `location@backend` syntax.
+
+```bash
+autorestic backup -l location@backend
+```