update doc/en structure

doc/en/assets-preload.md

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+# Assets preloading (*preload.json* file)
+
+`preload.json` in content-pack folder is used for specifying additional assets should be loaded, like sounds.
+
+The file contains following categories available:
+- fonts
+- shaders
+- textures
+- sounds
+
+> [!NOTE]
+> Sound loading with all variations following template:
+> (sound: *sound_name*) -> *sound_name.ogg, sound_name_1.ogg, sound_name_2.ogg, ...*
+> or *sound_name_0.ogg, sound_name_1.ogg, sound_name_2.ogg, ...*
+
+Adding sound `packid:sounds/events/explosion.ogg` with all variants example:
+```json
+{
+	"sounds": [
+		"events/explosion"
+	]
+}
+```
+
+Sound will be available as "events/explosion"
+
+Additional load settings example:
+```json
+{
+    "sounds": [
+        {
+            "name": "events/explosion",
+            "keep-pcm": true
+        }
+    ]
+}
+```
+
+*preload.json* example from `core:` package (`res/preload.json`):
+```json
+{
+    "shaders": [
+        "ui3d",
+        "screen",
+        "background",
+        "skybox_gen"
+    ],
+    "textures": [
+        "misc/moon",
+        "misc/sun",
+        "gui/crosshair"
+    ]
+}
+```
+