Updated release wiki page

[mcatanzaro gnome org]

Hi release team,

I've largely rewritten this wiki page:

https://wiki.gnome.org/ReleasePlanning/MakingARelease

with the goal of making it much simpler and easier to follow. I've also 
emphasized the process of building the entire core.bst project before 
uploading anything. References to smoketesting are removed because we 
haven't done that in several years.

Feedback welcome,

Michael