Write SQL query to solve given problem: What is the percentage of documentary films?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT CAST(SUM(IIF(T2.name = 'Documentary', 1, 0)) AS REAL) * 100 / COUNT(T1.film_id) FROM film_category AS T1 INNER JOIN category AS T2 ON T1.category_id = T2.category_id