devdocs notes on memory layout and runtime initialisation

Some notes I made while learning about runtime internals.
Maybe useful for other new developers.
But maybe misleading because eggnog :)

this file really should be comments on the code itself. there are too many specific implementation details mentioned which may change, and this file will then get out of sync. moving these comments to the function themselves would help avoid that. and the url links themselves will become out-of-sync almost immediately.