score/time_slave: Create detailed design (comp_dd_sta)

> [!IMPORTANT]
> Link the implementing PR to this issue by including `Closes #88` in the PR description or commit message.

### What

Write the detailed design document for `score/time_slave`.

Create `score/time_slave/docs/detailed_design/index.rst` using the sphinx-needs `comp_dd_sta` directive (the docs folder exists but is empty). Decompose the component into units, document their responsibilities and interfaces, and connect every unit to requirements (`:implements:`) and architecture (`:satisfies:`). Cover both Linux and QNX platform variants explicitly.

> **Note**: This ticket targets v0.10.

Units to cover: gPTP adapter (interfaces with gPTP daemon), PHC writer (adjusts system clock via PHC), platform abstraction layer (Linux vs QNX variants), application lifecycle.

### Acceptance Criteria

- [ ] `comp_dd_sta` directive in place with `:status: valid`
- [ ] Platform variants (Linux/QNX) documented
- [ ] `:implements:` and `:satisfies:` links present for every unit
- [ ] `req-Id` traceability comments in source for every traced unit

### References

- [S-CORE Implementation & Detailed Design](https://eclipse-score.github.io/process_description/main/process_areas/implementation/)