Revert "Remove page.php"

This reverts commit 607d5d2b941b7377da34f5bbdbb5fb0dbede674c.
Revert "Restore upgrade-guide files so cruelly left behind."

This reverts commit 213eff697255470361920ca03a37d41c0137dae0.
jquery.com: Restore jq-primaryContent support. Fixes upgrade guide.