Add info about SR/pp algorithms

[cl8n]

pp only has osu!standard stuff (and is probably outdated), star rating has nothing

[Gamingzone123]

probably best for smoogi or someone else heavily involved in the pp changes to do this, star rating should just be a matter of looking into lazer's code

[peppy]

The better way of doing this would likely be to go through all the star rating / pp update news posts and summarise/extract the relevant pieces. Things have gotten a lot more complicated over the last few years, and I can agree we should do a better job at getting all the information in one place.