This ticket covers the following improvements to Mongoid's installation/compatibility documentation:

Move compatibility tables from "Rails considerations" to "Installation" page, so that they are together with the Ruby compatibility table and are found earlier in the documentation where users are more likely to look for them.
Redo compatibility tables in the style used in the driver, where Mongoid versions are on the left going down and the versions of the other component are going right. The existing matrix used for Ruby and MongoDB versions concurrently has a lot of data but does not provide any historical information, and as such is of rather limited value.
Move the brief Rails-related installation instructions we have from the Rails considerations page to the installation page, where they would be more discoverable.
Add MongoDB 4.2 to the server compatibility table.
Add a compatibility table for the driver.