Edits to product page to define Backlog and remove redundancy

Merged Ernst van Nierop requested to merge evn-remove-backlog into master

This came up by a conversation in Slack where we were trying to figure out what Backlog meant. Since it is ambiguous, why not just drop it (update: not dropping it). For reference: https://gitlab.slack.com/archives/C1CMPT031/p1501867934422614?thread_ts=1501866613.759621&cid=C1CMPT031

While I was at it, I noticed some duplicate information and attempted to remove it.

FWIW, the UX team and Frontend teams still make explicit mention of the Backlog milestone as something to look for in picking what to work for. So that should then either be removed in their workflows, or perhaps we end up discarding this merge request in favor of one that specifically defines what Backlog is and what belongs there.

\cc @lbot @mydigitalself @joshlambert @tauriedavis

Edited by Ernst van Nierop