Update teach.md
Update teach.md

providing a landing page for Teach

Former-commit-id: 6e7b008d27a293c41bcf40dd5ef5f9e9ed223bb0 [formerly 634683dd45d57e17a031d9b31565af7933997312]
Former-commit-id: 42f1f9437e28858567a0fe3ee0def9cf77a79585
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io