feat: #3630579 Use declare(strict_types=1) in new code

Issue: #3630579: Use declare(strict_types=1) in new code

Proposed changes

Update docs/php/coding.md to require declare(strict_types=1); in all new PHP files following the approved Drupal core policy #3250827.

Also update the code examples to demonstrate the declare statement surrounded by empty newlines (for files both with and without a file-level DocBlock).