Docs: link accessibility roadmap from Goal.md

[sbrants]

Summary

docs/accessibility.md tracks a11y phases (done and planned). Goal.md is the main product spec but does not yet link to it (see Phase E in the accessibility doc).

Task

1. Add a short Accessibility section to Goal.md (target: pragmatic WCAG AA for core flows, not a formal certification).
2. Link to docs/accessibility.md and note that charts are supplementary; table fallback is planned (Phase C).
3. Keep the section brief — a few bullets, not a duplicate of the full roadmap.

Why

Helps contributors and users find a11y work without searching the repo. Pure documentation; no app code required.

References

• docs/accessibility.md — Phase E
• CONTRIBUTING.md