ci: check relative Markdown links and anchors #563

Summary

Nothing in CI checks the repository's relative Markdown links. #529 moved the design documentation, numbered its sections, and retargeted every inbound link by hand, verified with a one-off script. That script also found an older broken link: line 942 of aci_edge_sandboxes/README.md links #native-proxy-and-forwarded-ports, but #512 renamed that heading to "Native proxy and host loopback".

Why

• Section anchors now carry numbers. Documents link into the architecture guide through anchors such as 3.2-clock-and-time.md#3210-guest-obligations and 5.1-boot-protocol.md#511-linux-direct-loader, from doc/benchmarks.md, doc/build.md, doc/ci.md, doc/run.md, doc/usage.md, and doc/guests/. Renumbering or renaming a section breaks them silently: GitHub opens the page at its top.
• Code cites documents by path. After doc: replace doc/design with an architecture-only design guide #529 removed doc/design/time-abi.md, the comments that cited it had to be found by search; time ABI: retarget citations of the removed specification and rehome its reference detail #562 tracks the citations that remain. A check of the doc/ paths in tracked files would have listed them.
• The quality job covers documentation-only changes. Lint, typing, and formatting runs for every non-draft pull request, including documentation-only ones, which skip the workloads, so a check in check-quality runs on the changes most likely to break a link.

Tasks

• Fix the anchor on line 942 of aci_edge_sandboxes/README.md.
• Add a step to .github/actions/check-quality/action.yml that resolves every relative link and image target in the tracked Markdown files outside openvmm/, and every anchor against GitHub's heading slugs, ignoring fenced and inline code. A small script under scripts/ stays under ruff and pyright and needs no new dependency.
• Optionally, also check the doc/ paths that tracked files mention, with an allowlist for test fixtures such as doc/notes.md in scripts/test_nvx_tools.py.