cha-config(5) or about:chawan. The former also has an online version: https://chawan.net/doc/cha/config.html#pager-actions

For navigation in particular you'd use capital D to discard the current buffer and return to the previous page. There's also , (comma, back) and . (period, forward), which non-destructively cycle through the stack.

(Well, it's really a tree, but the UI mostly treats it as a stack.)