Add the information of necessary intermediate releases for users upgrading from very old releases to the latest to the upgrading guide

I am currently helping customers to upgrade from 7.x or 8.x to upgrade to the latest. It turns out we should follow some pattern for intermediate versions to get the whole process run smoothly. We should include this into the main update document.

Furthermore for 9.x to 10.x specific, it's required to stay with 9.5 for some time until the background migrations are done. Otherwise some data will be missing in 10.x. This is mentioned in the upgrade barometer of 9.5 which should also be included in main update document so that users could have a central place to better understand the whole process.