A9.com was taken down 2019, and with it all the official opensearch
documentation. This commit updates all the links to a webarchive version. There
also is a archived version at https://github.com/dewitt/opensearch but they
also rely on webarchive for rendering so I decided to reference the original
A9.com pages.
This commit deliberatley left the xmlns paths unchanged.
Differential Revision: https://phabricator.services.mozilla.com/D241105
114ab195a4