Seaside - Add manual installation instructions

[dmitrizagidulin]

Currently, the Seaside chapter recommends to use the Seaside One-Click Experience. (In fact, all the Seaside documentation does this. I haven't been able to easily find any install instructions at all, beyond the obsolete One-Click Images).
Unfortunately, those images not only have an old version of Seaside, but are based on Pharo 1.4.

Since this is a Pharo 4+ book, we need to include actual installation instructions for Seaside.

(Part of issue #126)

[dmitrizagidulin]

It turns out that there are updated One-Click experience images, for Pharo 4 + Seaside 3.1!
I updated the installation instructions to use them.
Still need to add manual installation instructions, however.