docs(webvh): add WebVH issuer onboarding roadmap and remove plan doc

[PatStLouis]

Summary

Add the WebVH Issuer Onboarding Roadmap and remove the planning document. Addresses #275.

What changed

• Added WebVH Issuer Onboarding Roadmap — stage-by-stage onboarding for BC Gov issuers (provincial + municipal) with:
  • Two paths: new issuer (Path A) and Indy migrant (Path B)
  • DITP owns ACA-Py agent; issuer owns controller only; target state: migrate from single instance toward Traction tenant
  • Stages 0–5 (pre-reqs, infra, integration, dual issuance for Path B, production, steady state), timelines, and appendices/links
• Removed docs/webvh-issuer-onboarding-roadmap-plan.md (planning doc; decisions folded into roadmap)
• Updated docs/README.md — roadmap link only; removed plan link

Scope

• Audience: BC Gov only (provincial and municipal)
• WebVH server and witness: always DITP-provisioned shared/platform
• Governance doc to be linked in roadmap when available

[esune]

We can likely use https://developer.gov.bc.ca/docs/default/component/digital-trust/governance/service as URL to link for the governance documents hosted on DevHub (I think this is the right path based on the base url and structure).