docs.dagger.io/install sends to deprecated 0.2 docs

[shykes]

Problem

When navigating to https://docs.dagger.io/install I see the old 0.2 install docs, without context making that clear. There are 2 problems here:

• The URL doesn't have an ID so it implies that it's "mutable" and kept up-to-date
• Lack of context makes the confusion worse

Solution

Reclaim "intuitive" URLs such as /install to always serve up-to-date contents; and/or add a banner on out-of-date content so that it's clear to the user.