Improvement: Enhance internal-first workflow documentation with detailed process and diagram

### What

## Problem

The current internal-first workflow documentation lacks detail and visual representation, making it difficult for contributors to understand the complete process flow between internal and public repositories.

## Proposed Solution

Improve the documentation in `docs/contribute/fork/index.md` to include:

### Detailed Workflow Steps
- Clear step-by-step process from internal development to public contribution
- Explanation of review and validation checkpoints
- Boundaries between company internal and public spaces

### Visual Diagram
- Create a workflow diagram showing the complete process
- Illustrate repositories: company-internal → company-contrib → eclipse-score
- Show actors: Developer, Reviewer, automated checks
- Highlight security boundaries and validation steps

### Specific Content
1. Development in `company-internal/feature_unverified` (branched from `eclipse-score/main`)
2. Pull request to `company-internal/feature_verified`
3. Internal review with automated checks (e.g., secret scanning)
4. Transfer to `company-contrib/feature` (manual or automated)
5. Open PR to `eclipse-score/main`

## Acceptance Criteria

- [x] Add detailed workflow steps with clear numbering
- [x] Create visual diagram (SVG format) showing the complete flow
- [x] Document the boundaries between internal and public development

## Benefits

- Better contributor onboarding for internal-first approach
- Reduced confusion about repository boundaries
- Clear understanding of security requirements
- Improved contribution process efficiency

### How

_No response_

### Estimates for realization

hours

### Category

- [ ] Affects Detailed Design

### Requirements / Architecture

- [x] Requirements / Architecture are not affected by this change?