Add documentation on how to build on top of EESSI as a site - and include the use of `EESSI_SITE_SOFTWARE_PREFIX` in that

[casparvl]

We should have documentation on our recommended ways to build on top of EESSI as a site. One thing that should be covered is how to use EESSI_SITE_SOFTWARE_PREFIX to control the location of where these installations end up (this was introdcued in EESSI/software-layer-scripts#232).

[Neves-P]

This gets fixed by #778, right?

Yes