Reorganize proposal with Overview section and better flow

Restructure the document to improve readability and follow logical
progression from high-level to detailed design:

Changes:
- Add 'Overview' section with high-level description and architecture flow
- Move architecture flow diagram from buried section to Overview (first thing readers see)
- Rename 'API Design' subsections for clarity:
  - 'Architecture Components' → 'Component Overview'
  - Group detailed flows under 'Implementation Details'
- Add forward reference link to API Specification
- Remove duplicate High-Level Architecture diagram
- Promote 'Architecture and Components' to top-level subsection
- Promote 'Implementation Details' to top-level subsection

New flow:
  User Stories → Overview (what/why) → API Design (examples) →
  Architecture (components) → Implementation (how) → Security → Risks

This matches the recommendation: show WHAT before HOW, with
progressive detail from overview to implementation specifics.

Signed-off-by: zhujian <jiazhu@redhat.com>

Author: zhujian7