fix: Return page width and height from document (#700)

* fix: Return page width and height from document

Some documents could have pages without MediaBox, at this case we need
to get the MediaBox from document and not from a specific page.

Signed-off-by: Vitor Mattos <vitor@php.rio>

* fix: prevent psalm error

Prevent the follow error:

```
ERROR: InvalidPassByReference - Parameter 1 of reset expects a variable (see https://psalm.dev/102)
				$details = reset($pdf->getObjectsByType('Pages'))->getHeader()->getDetails();
```

Signed-off-by: Vitor Mattos <vitor@php.rio>

* Update doc/Usage.md

Co-authored-by: Konrad Abicht <hi@inspirito.de>

---------

Signed-off-by: Vitor Mattos <vitor@php.rio>
Co-authored-by: Konrad Abicht <hi@inspirito.de>

Author: vitormattos

doc/Usage.md

@@ -224,6 +224,11 @@ $pages = $pdf->getPages();
 $mediaBox = [];
 foreach ($pages as $page) {
     $details = $page->getDetails();
+    // If Mediabox is not set in details of current $page instance, get details from the header instead
+    if (!isset($details['MediaBox'])) {
+        $pages = $pdf->getObjectsByType('Pages');
+        $details = reset($pages)->getHeader()->getDetails();
+    }
     $mediaBox[] = [
         'width' => $details['MediaBox'][2],
         'height' => $details['MediaBox'][3]