docs: remove the outdated roadmap

[loehub]

Pull Request

Summary

Removes the roadmap from the docs. It no longer reflects where the project is going, so it's better gone than left as stale plans in the published docs.

• Deletes docs/roadmap.rst.
• Removes it from the Architecture table of contents in docs/index.rst.
• Drops the Roadmap link from the README's Documentation section.

---

Issue

No linked issue. Suggested by the team in review chat.

---

Type

• Bug fix
• New feature or tests
• Breaking change
• Documentation only

---

Testing

• git grep -i roadmap finds nothing left in the repository.
• Nothing in selfpatch/website, the org profile, selfpatch_demos, ros2_medkit_web_ui, ros2_medkit_mcp or ros2_medkit_clients links to roadmap.html.
• Built the docs locally with the pinned Sphinx 8.2.3. There are no new warnings and no broken toctree entry. Doxygen and PlantUML weren't installed locally, so their existing warnings were expected.

---

Checklist

• Breaking changes are clearly described (and announced in docs / changelog if needed)
• Tests were added or updated if needed
• Docs were updated if behavior or public API changed