According to a Gartner study of 7,300 employees and managers, plus interviews and surveys with HR executives, the most effective managers are "connectors." That is, they give feedback when they can, and the rest of the time, they connect employees with other people who are better equipped to help them.
Connector managers are comfortable admitting to their own inadequacy at times; as the article says, "they recognise that many skills are best taught by people other than themselves."
The other three types of managers are teacher managers, who lead based on their own knowledge and experience; always-on managers, who provide continual coaching and feedback; and cheerleader managers, who are more hands-off when it comes to employee development.
Results showed that employees coached by connectors were three times as likely as employees coached by other types of managers to be high performers.
The article recommends that managers broaden conversations about employee development to the entire team, and that managers encourage colleagues to coach one another.
A blog post on the Gartner website says employees can identify connector managers by looking for people who "invest time to diagnose and understand individual employee needs" and who "help employees get more value from their development connections," as opposed to focusing on simply on creating bigger networks.
Another article on HBR, by Jon Younger, encourages managers to seek out seasoned workers in an organisation who can help more junior employees. He specifically mentions consultants and freelance workers, but there's no reason not to include willing full-time employees on other teams — basically anyone who doesn't have the skills you have. Younger recommends establishing informal coaching relationships and asking those experts to provide developmental feedback as well.
Bottom line: You don't have to be the only resource available to your employees. In fact, you'll be more effective if you help them find other resources who can help them get where they want to go.