Update Mass Module docs

[bretg]

The Mass module has been re-positioned as a general real rendering module. All of the massplatform links within this file are dead. Someone with knowledge of this module needs to update it to generalize.

[muuki88]

@lcorrigall do you have insights on this? From my personal history I remember IX supporting MASS. Maybe you have somebody that can shed some light on the docs here.

[patmmccann]

deleted mass as part of prebid/Prebid.js#9888, need the example in the docs if someone wants to do this