completely rework the persitence article

[Jorropo]

For #1798

My english should be reworked. The article is currently written as one character conversation which sounds weird, I think it would sound better if it were less conversational. There are lots of opportunities to shorten my writting.

I also nuked a lot of the pinning and filecoin explainations where are aside of the whys, this is probably usefull to keep somewhere.