chore: backups custom location and config file docs (#13996)

docs/docs/guides/custom-locations.md

@@ -17,6 +17,7 @@ In our `.env` file, we will define variables that will help us in the future whe
 + THUMB_LOCATION=/custom/path/immich/thumbs
 + ENCODED_VIDEO_LOCATION=/custom/path/immich/encoded-video
 + PROFILE_LOCATION=/custom/path/immich/profile
++ BACKUP_LOCATION=/custom/path/immich/backup
 ...
 ```
 
@@ -30,6 +31,7 @@ services:
 +     - ${THUMB_LOCATION}:/usr/src/app/upload/thumbs
 +     - ${ENCODED_VIDEO_LOCATION}:/usr/src/app/upload/encoded-video
 +     - ${PROFILE_LOCATION}:/usr/src/app/upload/profile
++     - ${BACKUP_LOCATION}:/usr/src/app/upload/backup
       - /etc/localtime:/etc/localtime:ro
 ```
 

docs/docs/install/config-file.md

@@ -35,6 +35,13 @@ The default configuration looks like this:
     "accel": "disabled",
     "accelDecode": false
   },
+  "backup": {
+    "database": {
+      "enabled": true,
+      "cronExpression": "0 02 * * *",
+      "keepLastAmount": 14
+    }
+  },
   "job": {
     "backgroundTask": {
       "concurrency": 5