The Ronald Reagan Presidential Foundation has quietly begun a nationwide effort to raise about $100 million to build a Reagan library and a public affairs center in the Stanford University area. Gary Jones, executive director of the foundation, said blue fund-raising packets bearing Reagan's signature and a color photo of the proposed library were sent out last month. He would not reveal how many of the brochures were sent or how much money has been raised so far. The foundation announced last year that it had set the $100-million fund-raising target. Two-thirds of the money would be for construction of the public affairs center and a partial endowment for its operation. Ten acres have been set aside at Stanford University for the library, which is to be built in the next two years at a cost of about $30 million. It will be operated by the federal government as part of the National Archives.