So man -k intro
gives a list including the following:
intro (1) - introduction to user commands
intro (2) - introduction to system calls
intro (3) - introduction to library functions
intro (4) - introduction to special files
intro (5) - introduction to file formats and filesystems
intro (6) - introduction to games
intro (7) - introduction to overview and miscellany section
intro (8) - introduction to administration and privileged commands
Each one can be read by man intro.N
(N being 1-8), these intro pages looks like the begining of chapters in the man documentation.