We need an upgrade page in the docs

[amychen1776]

Right now, when you're using an outdated version of dbt, the message that you get directs you to the installation page which no longer has a page on what commands you need to upgrade your adapter version to the latest core version.

[runleonarun]

I agree! @jasnonaz started a PR here, but I was hoping we could create a page for upgrades instead of just a section. What do you both think?