Add scenario-based documentation with decision flowchart

## Summary

Create scenario-level documentation that guides users through key decisions (branching strategy, source of truth, environment topology, etc.) via a decision flowchart, landing them in the appropriate "how-to" doc for their chosen setup.

## Problem

Users come to APIOps CLI with different organizational constraints and preferences. Currently, they must read through all documentation to figure out which setup applies to them. A guided decision tree would dramatically reduce time-to-value.

## Proposed Content

### Decision Flowchart
A visual flowchart (mermaid diagram or similar) that walks users through key decisions:

1. **Source of Truth** — Is APIM the source of truth, or is the Git repo?
2. **Branching Strategy** — Trunk-based, GitFlow, environment branches, etc.
3. **Environment Topology** — One APIM instance per environment, or multiple environments on a single instance?
4. **CI/CD Platform** — GitHub Actions or Azure DevOps?
5. **Change Flow** — Portal-first (extract → commit → promote) or code-first (edit → PR → publish)?

### Scenario Landing Pages
Each leaf of the decision tree links to a dedicated "how-to" page covering:

- Recommended repo structure
- Configuration file setup (filters, overrides)
- CI/CD pipeline configuration
- Step-by-step walkthrough for the chosen scenario
- Common pitfalls and FAQ

### Example Scenarios
- **Scenario A**: Git as source of truth, trunk-based development, separate APIM per environment, GitHub Actions
- **Scenario B**: Portal-first, feature branches, single APIM instance, Azure DevOps
- **Scenario C**: Hybrid (portal for discovery, Git for promotion), environment branches

## Acceptance Criteria

- [ ] Decision flowchart is created and embedded in documentation
- [ ] At least 3 scenario landing pages are written
- [ ] Each scenario page includes repo structure, config setup, and CI/CD guidance
- [ ] Flowchart is maintained as a mermaid diagram (or similar) for easy updates
- [ ] Documentation index/nav links to the scenario guide prominently