Merged the mission page into the about page

The mission was being restated on a third page, which meant three copies of the same paragraph to keep in agreement.

It now lives in one place, in the about page. The old address redirects there rather than disappearing, because a link that used to work should keep working. The merge happened while the site was being built and is written up here on the day it was noticed to be missing from this log — the log promised that withdrawn pages are recorded, and this one was not.