Add quickstart introduction to repositories & archives (fixes #3620)

(cherry picked from commit 09beb64f344c4b21f8a212f49ab6998ff4fbdc22)

docs/quickstart.rst

@@ -13,6 +13,19 @@ A step by step example
 
 .. include:: quickstart_example.rst.inc
 
+Archives and repositories
+-------------------------
+
+A *Borg archive* is the result of a single backup (``borg create``). An archive
+stores a snapshot of the data of the files "inside" it. One can later extract or
+mount an archive to restore from a backup.
+
+*Repositories* are filesystem directories acting as self-contained stores of archives.
+Repositories can be accessed locally via path or remotely via ssh. Under the hood,
+repositories contain data blocks and a manifest tracking which blocks are in each
+archive. If some data hasn't changed from one backup to another, Borg can simply
+reference an already uploaded data chunk (deduplication).
+
 Important note about free space
 -------------------------------