Missing pp_extras in net documentation

The doc page discussing reaction networks:

https://docs.mesastar.org/en/latest/net/nets.html

...references the pp_extras reaction, but that reaction isn't actually specified on the page. Needs to be fixed.

Open the reaction-network documentation page at docs.mesastar.org/en/latest/net/nets.html and locate the reference to the pp_extras reaction. Update the page so that pp_extras is actually specified, then verify that the rendered documentation no longer contains the inconsistency.

Written by the indexing model from the issue text.