Neovim's ":Man" command also shows a ToC when you type "gO" (mnemonic: "outline").

See also:

    :help gO

Currently Nvim builds this ToC with its own hacks, so it would be nice to get that info from mandoc instead. But IIUC, "-O toc" only works with HTML output of mandoc? How can non-HTML tools get the ToC from mandoc?