chore(CRP-2830): add canister READMEs

Author: fspreiss

backend/mo/canisters/ic_vetkeys_encrypted_maps_canister/README.md

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+# ic-vetkeys-encrypted-maps-canister
+
+The canister implemented in this folder directly exposes the methods of the encrypted maps.
+This is useful for:
+
+1. running canister tests
+2. implementing dapps that only require encrypted maps

backend/mo/canisters/ic_vetkeys_manager_canister/README.md

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+# ic-vetkeys-manager-canister
+
+The canister implemented in this folder directly exposes the methods of the key manager.
+This is useful for:
+
+1. running canister tests
+2. implementing dapps that only require a key manager

backend/rs/canisters/ic_vetkeys_encrypted_maps_canister/README.md

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+# ic-vetkeys-encrypted-maps-canister
+
+The canister implemented in this folder directly exposes the methods of the encrypted maps.
+This is useful for:
+
+1. running canister tests
+2. implementing dapps that only require encrypted maps

backend/rs/canisters/ic_vetkeys_manager_canister/README.md

@@ -0,0 +1,7 @@
+# ic-vetkeys-manager-canister
+
+The canister implemented in this folder directly exposes the methods of the key manager.
+This is useful for:
+
+1. running canister tests
+2. implementing dapps that only require a key manager