Hello,

The following binds "p" to backward-button, and "n" to forward-button in
help-mode-map for more conveniently jumping between buttons in *Help*
buffers.  I have found myself constantly reaching for these in *Help*
buffers after using helpful [0] for a while.

[0]: https://github.com/Wilfred/helpful

I thought about adding these to button-map itself, but that seemed like
a more invasive change, and I wasn't sure about its side effects e.g. in
non-read-only buffers.

The patch is against emacs-27, since it's a trivial and hopefully
completely harmless change.