[doc] orphaned Prerequisites section

[dariavladykina]

File: asciidoc/quickstart/eib.adoc

This section is orphaned as a single top-level item. I believe the entire structure must be updated to make several same-level chapters (e.g. Prerequisites, Getting the EIB image, Creating the image configuration directory, and so on) with their respective subordinate sections.

[agracey]

I moved everything down a level to where I think it flows better