Add Galaxy migration doc

A documentation for the future migration to UseGalaxy.fr

TODO

• repair FTP access on usegalaxy.fr
• add image for last history import step
• test the import of a big archive from the cluster
• review the text
• check schedule is ok
• check .fr data policy is up-to-date
• send email to genouest users
• send email to bipaa users
• add warning on welcome pages