Add scripts/validate_doc_links.py — scans all markdown files in the
repository, extracts inline and autolinks, and verifies each URL via
HTTP HEAD request (with GET fallback for servers that reject HEAD).
Features:
--root : repository root to scan (default: repo root)
--fail-on-broken : exit 1 if any broken links found
--json : emit JSON report for CI consumption
--ignore : comma-separated URL prefixes to skip
Ignores non-HTTP URLs, localhost/127.0.0.1, and private IP ranges.
Requires only Python stdlib — no external dependencies.
Smoke-tested against this repo: 2 unique URLs checked, 0 broken.
Addresses 4.8: Doc Link Validator acceptance criteria.
Closes#103