Rename "retrace" to "replay" within the main page.

One case has always been using "replay", but change this "retracing"
to "replaying" to match our latest convention.
Rename image from retrace.png to replay.png

A filename on the website is not a very significant user-facing feature,
but we might as well be consistent.

(If nothing else, this makes it easy to grep over the index.html file
and ensure that everything has been updated from "retrace" to
"replay".)
Link to Carl's presentation.

Note some misc links for future.