Replaced with discuss.html in various long-term contents pages.

I didn't fix in a few places on purpose:
- Old news articles because past is past and the old links will still
  work anyway (a redirection was set up in oscp branch), because good
  URLs don't change. So it's all good.
- Old userfaq for same reason. Outdated and all.
- file-list.md and wtf-pelican.md because these seem to be about the
  move to Pelican years ago, like a historical log of what was done. It
  makes no sense to touch these.