Add redirects for the showcase and meetings pages

[maxrjones]

Pangeo showcase is currently at https://pangeo.io/pangeo-showcase.html and meetings is currently at https://pangeo.io/meeting-notes.html. We should redirect to the new pages.

[weiji14]

The Pangeo Community Meeting calendar invite is currently pointing to https://pangeo.io/pangeo-showcase.html (gives a 404 page) instead of https://www.pangeo.io/showcase, we would need to update the link on gcal or set-up the redirect.