FRANKFURT, Germany — A leak of 11.5 million documents from a Panama-based law firm offers a glimpse into the shadowy world where the rich and powerful hide their money, raising sharp questions about the use of shell companies that obscure the identities of their true owners. Leaders of the Group of 20 — representing about 80 percent of the global economy — have vowed to crack down on the practice, which is blamed for helping conceal money laundering, corruption and tax evasion. By themselves, shell companies aren’t illegal.