so you're looking for a pretty lowlevel introduction to silverstripe on code-level, right?
everything starts at sapphire/main.php, and then you can work your way onwards through Director and the sapphire/core directories. most of these are documented already:
http://doc.silverstripe.com/doku.php?id=execution-pipeline
http://doc.silverstripe.com/doku.php?id=director
http://doc.silverstripe.com/doku.php?id=datamodel
what would you expect from the documentation you're looking for?
does the proposed "learning path" on doc.silverstripe.com suit you?