doc: sframe: segregate the ABI/arch-specific components

doc: sframe: segregate the ABI/arch-specific components

The recipe to interpret the SFrame FRE stack offsets is
ABI/arch-specific.

Although, there is other information in the specification that is
ABI-specific (like pauth_key usage in AArch64), those pieces of
information are now assimmilated in the SFrame specification in a way
that it is fairly difficult to carve then out into a ABI/arch-specific
section without confusing the readers.

For future though, the specification must strive to keep the generic
parts and ABI/arch-specific parts clearly laid out in separate sections.

libsframe/
	* doc/sframe-spec.texi: Reorder and adapt the contents.