docs: add Platform header to browser support tables

The website's swizzled MDX Table component (packages/website/src/theme/Table.js)
strips the thead when any header cell is empty, hiding the browser column
headers. Labeling the first column restores them on the docs site.

Author: ilhan007

docs/11-Browser-and-Platform-Support.md

@@ -9,14 +9,14 @@ UI5 Web Components are supported by all major modern browsers on the desktop and
 
 ## Desktop
 
-|                                | Google Chrome   | Microsoft Edge  | Mozilla Firefox | Mozilla Firefox ESR | Apple Safari    |
+| Platform                       | Google Chrome   | Microsoft Edge  | Mozilla Firefox | Mozilla Firefox ESR | Apple Safari    |
 | ------------------------------ | --------------- | --------------- | --------------- | ------------------- | --------------- |
 | **Windows** (versions 10 & 11) | Latest 2 Stable | Latest 2 Stable | Latest 2 Stable | Latest              | n/a             |
 | **macOS** (Latest 2 versions)  | Latest 2 Stable | Latest 2 Stable | Latest 2 Stable | Latest              | Latest 2 Stable |
 
 ## Mobile
 
-|                                   | Google Chrome   | Apple Safari    |
+| Platform                          | Google Chrome   | Apple Safari    |
 | --------------------------------- | --------------- | --------------- |
 | **Android** (Latest 3 versions)   | Latest 2 Stable | n/a             |
 | **iOS & iPadOS** (Latest version) | -               | Latest 2 Stable |