Finalize extension documentation

php_modules/pdo/README.md

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+# pdo
+
+The PHP Data Objects (PDO) extension defines a lightweight, consistent interface for accessing databases in PHP. Each database driver that implements the PDO interface can expose database-specific features as regular extension functions. Note that you cannot perform any database functions using the PDO extension by itself; you must use a database-specific PDO driver to access a database server.
+
+| Platform | Url                                                              |
+|----------|------------------------------------------------------------------|
+| PHP.net  | https://www.php.net/manual/en/book.pdo.php                       |