where: Revise Choosing an Image

[ziaee]

Summary

Streamline by feeing Archives and Architectures to the introduction.

Discussed with: cperciva, dch, linimon

[ziaee]

Here's how it renders on my laptop:

[carlavilla]

For the section "Choosing an Image", I prefer to go with this one: https://reviews.freebsd.org/D57051
I think the change in this review is too much.
The other changes, I am ok.

Let me commit the other review with the change proposed by dch@

[ziaee]

Abandoned in favor of https://reviews.freebsd.org/D57051, other parts of this commit to be folded into https://reviews.freebsd.org/D57165.