docs(tvix/castore): fix tvix_castore::import sub-mod docstrings

The one for `fs` was wrong, and ended up being attached to ingest_path,
and the one for `archive` was missing entirely.

Change-Id: I8a4c32fb5293badb1ea0764c278a88e4ca33c018
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11552
Tested-by: BuildkiteCI
Reviewed-by: edef <edef@edef.eu>

tvix/castore/src/import/archive.rs

@@ -1,3 +1,5 @@
+//! Imports from an archive (tarballs)
+
 use std::io::{Cursor, Write};
 use std::sync::Arc;
 use std::{collections::HashMap, path::PathBuf};