Rename the ISA spec file
This is to facilitate including other relevant specifications here, namely a system architecture to surround the
instruction set.

Specified the behaviour of hardware interrupts.

Renamed the specification document, and minor tweaks.
Changed the versioning to SemVer since it was already close to it, and fixed some capitalisation issues.

Write instruction descriptions.
In particular, this specifies the operand positions and usages.

Second pass for overview and instruction categories.
Made the overview easier to read by grouping details more sensibly, and made the descriptions
in the instruction categories more consistent.

Initial Commit
Add first and second draft of spec and unfinished first version