docs(manual): cross-cutting chapters, signpost the templates and container READMEs, delete MIGRATION-GUIDE and CONTAINER-GUIDE

[quantecon-services]

PR 4 of the user manual (#178). See #178, "What the manual covers" and decision 4.

• Chapters under docs/user:
  • Getting started, including "Migrating an existing repo";
  • How the actions compose;
  • Containers;
  • Versioning;
  • Security.
• templates/README.md and the three container READMEs become signposts. The container READMEs' Development, Automated Builds and Testing sections move to docs/dev.
• Delete docs/MIGRATION-GUIDE.md and docs/CONTAINER-GUIDE.md. The per-repo migration notes move to PLAN's rollout section.
• Point the root README's Documentation section and CONTRIBUTING's doc-update matrix at the manual.