docs: remove committed spec.md; generate /spec/ from canonical ard-spec

The spec no longer lives in this repo at all. The build hook now injects the
/spec/ page as a generated file fetched from ards-project/ard-spec at build
time (with link rewrites), falling back to a short pointer page if unreachable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rvguha