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 :)

even though this struct does not exist in a C header file, it is still well-defined. every julia type lives a double-life as a boxed and unboxed version, although #2818 should simplify this greatly (by removing the `struct jl_value_t *type` field from the struct definition. regardless, the padding is not part of the type, but part of the allocator. in #8134, we could allocate a tighter box for this type.

(the alignof and sizeof this struct is operating system, not word size, dependent)