docs(decisions): add DR-001 for unit test design

[clanghans]

Summary

• Adds docs/decisions/ directory with an index and the first Decision Record
• DR-001-Infra documents four design decisions from the unit test infrastructure work in PR feat(test): add unit test infrastructure with py_itf_unittest #94:
  1. Dedicated py_itf_unittest macro (thin wrapper, no plugin machinery)
  2. Surgical Bazel target splitting for atomic dependency scoping
  3. Shared main.py bootstrap pattern for pytest under Bazel
  4. pytest-mock over unittest.mock
• Naming convention and index structure follow the Eclipse S-CORE DR convention (DR-{number}-{category}.md, glob-based toctree per category)
• Wires the new decisions/index into the Sphinx toctree

Related: #94
Discussion: https://github.com/orgs/eclipse-score/discussions/2867

Test plan

• Sphinx build succeeds (bazel build //docs)
• DR renders correctly in the generated docs site
• Links to PR feat(test): add unit test infrastructure with py_itf_unittest #94 and the GitHub discussion are reachable

[Copilot]

Pull request overview

Adds a new “Decisions” documentation section to capture design decision records (DRs) for ITF, starting with DR-001 describing the unit test infrastructure decisions introduced in PR #94.

Changes:

• Add docs/decisions/ with an index page and the first decision record (DR-001-infra).
• Wire the new decisions section into the main Sphinx docs toctree.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
docs/index.rst	Adds decisions/index to the site-level toctree so the section appears in the docs build.
docs/decisions/index.rst	Introduces the Decisions landing page and a globbed toctree for infra DRs.
docs/decisions/DR-001-infra.md	Documents the rationale and selected options for the unit test infrastructure design from PR #94.

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