Add a last updated label on the doc pages.

[Fanna1119]

with the speed amplify changes and gets new features, it is frustrating jumping around the docs, not knowing how up to date it is.

[timngyn]

Hi @Fanna1119, thank you for opening this issue! docs.amplify.aws now has when a page was last updated. It was merged in from this PR: #4341

It's located in the left menu and looks like this: