[prdoc] Topic support

[ggwpez]

We got feedback from marketing and non-Parity devs that it is often difficult to judge for outsiders what topic a MR is advancing.

Another issue: our changelog cannot be sorted by topics. It would be nice to have one section for Coretime, Bridges, one for Elastic Scaling or whatever it is. This would make it easier for the reader to see what we are pushing.

One way to address this would be add topics to the Prdoc. The "topic" would basically be a project. Sure there will be a misc section or something to opt-out. I am opening this issue here to gauge interest.

[bkchr]

topic field already exists.