[hub75] Add set_brightness action (#5792)

* [hub75] Add set_brightness action

* Fix template link

* address review comments

Author: stuartparmenter

content/components/display/hub75.md

@@ -306,6 +306,24 @@ The three key settings for LVGL are:
 - `auto_clear_enabled: false` - LVGL handles clearing
 - `double_buffer: false` - LVGL manages its own buffering
 
+## Actions
+
+### `hub75.set_brightness` Action
+
+This action allows you to dynamically change the brightness of the display at runtime.
+
+```yaml
+on_...:
+  - hub75.set_brightness:
+      id: matrix_display
+      brightness: 128
+```
+
+**Configuration variables:**
+
+- **id** (*Optional*, [ID](/guides/configuration-types#id)): The ID of the HUB75 display component. Only required if you have multiple `hub75` platform displays configured.
+- **brightness** (**Required**, int, [templatable](/automations/templates)): The brightness level to set (0-255). A value of 0 effectively turns off the display, while 255 is maximum brightness.
+
 ## Configuration Examples
 
 ### Basic Single Panel (with Board Preset)