docs: add detailed single-region onboarding and separate technical reference

[Hou (SciencePotato)]

Summary

• Make the main README a detailed, explained single-region onboarding guide with the single-region diagram from page 1 of the supplied architecture PDF.
• Explain prerequisites and permissions, required inputs, provider/app registration, language selection, deployment approval and outputs, provider authentication, validation, manual activation/rollback, and a completion checklist.
• Preserve the previous technical README in TECHNICAL.md and update documentation links.
• Keep multi-region and resiliency guidance out of scope.

Compatibility

• Rebased onto the latest main, preserving current Key Vault certificate issuance and background credential refresh documentation.
• Documentation and one diagram only; no application, infrastructure, or pipeline changes.

Validation

• Verified local Markdown links and heading anchors across all six changed documentation files.
• Verified TECHNICAL.md retains the latest main README content apart from title/navigation updates.
• Confirmed the diagram exactly renders page 1 of the user-updated PDF (October 5); no label edits were applied. Only the single-region image is embedded. Visually checked the rendering; the multi-region page is excluded.
• git diff --check passed. Application tests were not run because this is documentation-only.

[siyixian]

Provider adapter is part of SendOtp api, let's put them into same box