Fix examples/config-sample.json to match the current config format

The sample config has not been valid since the 2023 disk layout rework
and currently fails to parse, so the file the README points users at
cannot be used:

- sector_size was null, but SectorSize became a required object
- partitions were missing the dev_path key, which the parser reads
- the /home size used the Percent unit, which no longer exists
- /home started at 20 GiB while / ended at 20993 MiB, overlapping it
- gfx_driver kept the old "All open-source (default)" value

Sizes and keys now mirror what the installer itself writes when saving
a configuration.

examples/config-sample.json

@@ -17,12 +17,16 @@
         "partitions": [
           {
             "btrfs": [],
+            "dev_path": null,
             "flags": [
               "boot"
             ],
             "fs_type": "fat32",
             "size": {
-              "sector_size": null,
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
               "unit": "MiB",
               "value": 512
             },
@@ -30,7 +34,10 @@
             "mountpoint": "/boot",
             "obj_id": "2c3fa2d5-2c79-4fab-86ec-22d0ea1543c0",
             "start": {
-              "sector_size": null,
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
               "unit": "MiB",
               "value": 1
             },
@@ -39,10 +46,14 @@
           },
           {
             "btrfs": [],
+            "dev_path": null,
             "flags": [],
             "fs_type": "ext4",
             "size": {
-              "sector_size": null,
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
               "unit": "GiB",
               "value": 20
             },
@@ -50,7 +61,10 @@
             "mountpoint": "/",
             "obj_id": "3e7018a0-363b-4d05-ab83-8e82d13db208",
             "start": {
-              "sector_size": null,
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
               "unit": "MiB",
               "value": 513
             },
@@ -59,20 +73,27 @@
           },
           {
             "btrfs": [],
+            "dev_path": null,
             "flags": [],
             "fs_type": "ext4",
             "size": {
-              "sector_size": null,
-              "unit": "Percent",
-              "value": 100
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
+              "unit": "GiB",
+              "value": 10
             },
             "mount_options": [],
             "mountpoint": "/home",
             "obj_id": "ce58b139-f041-4a06-94da-1f8bad775d3f",
             "start": {
-              "sector_size": null,
-              "unit": "GiB",
-              "value": 20
+              "sector_size": {
+                "unit": "B",
+                "value": 512
+              },
+              "unit": "MiB",
+              "value": 20993
             },
             "status": "create",
             "type": "primary"
@@ -138,7 +159,7 @@
     "parallel_downloads": 5
   },
   "profile_config": {
-    "gfx_driver": "All open-source (default)",
+    "gfx_driver": "All open-source",
     "greeter": "sddm",
     "profile": {
       "details": [