alias: add new backend to create aliases for remote names #1049

The alias backend is a wrapper for an existing remote.
It allows you to name a "remote:path" as an "alias:".

bin/make_manual.py

@@ -21,6 +21,7 @@ docs = [
     "overview.md",
 
     # Keep these alphabetical by full name
+    "alias.md",
     "amazonclouddrive.md",
     "s3.md",
     "b2.md",