move menu pages loader to Lua & add gui.load_document, file.ext(), file.prefix() & add core:gui_util module

doc/en/scripting/builtins/libfile.md

@@ -125,10 +125,22 @@ Combines objects from JSON files of different packs.
 file.name(path: str) --> str
 ```
 
-Extracts the file name from the path.
+Extracts the file name from the path. Example: `world:data/base/config.toml` -> `config.toml`.
 
 ``lua
 file.stem(path: str) --> str
 ```
 
-Extracts the file name from the path, removing the extension.
+Extracts the file name from the path, removing the extension. Example: `world:data/base/config.toml` -> `config`.
+
+```lua
+file.ext(path: str) --> str
+```
+
+Extracts the extension from the path. Example: `world:data/base/config.toml` -> `toml`.
+
+```lua
+file.prefix(path: str) --> str
+```
+
+Extracts the entry point (prefix) from the path. Example: `world:data/base/config.toml` -> `world`.

doc/en/scripting/builtins/libgui.md

@@ -79,3 +79,16 @@ gui.confirm(
 ```
 
 Requests confirmation from the user for an action. **Does not** stop code execution.
+
+```lua
+gui.load_document(
+    -- Path to the xml file of the page. Example: `core:layouts/pages/main.xml`
+    path: str,
+    -- Name (id) of the document. Example: `core:pages/main`
+    name: str
+    -- Table of parameters passed to the on_open event
+    args: table
+) --> str
+```
+
+Loads a UI document with its script, returns the name of the document if successfully loaded.