Add migration page for Gemnasium
What does this MR do?
Add a new documentation page for migrating from Gemnasium.
Are there points in the code the reviewer needs to double check?
none.
Why was this MR needed?
We need a page to document the migration path. This page will be the placeholder for Gemnasium.com starting May 15.
What are the relevant issue numbers?
Edited by Philippe Lafoucrière