rustdoc: add crate level doc

Contains a link to the 'backup' module's doc, as that explains a lot
about the inner workings of PBS and probably marks a good entry point
for new readers.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/lib.rs

@@ -1,3 +1,8 @@
+//! See the different modules for documentation on their usage.
+//!
+//! The [backup](backup/index.html) module contains some detailed information
+//! on the inner workings of the backup server regarding data storage.
+
 pub mod task;
 
 #[macro_use]