RUBY-2519 modernized documentation and commands, however it missed the documentation URL.

Assuming the choice is made to replace master with upcoming:

Update navigation (rename master (upcoming) to Upcoming)
Update URL schema (rename *://www.mongodb.com/docs/mongoid/master/ to *://www.mongodb.com/docs/mongoid/upcoming)
Update all *://www.mongodb.com/docs/mongoid/master/ to 301 to *://www.mongodb.com/docs/mongoid/upcoming/