docs: add architecture decision records page

[stanlrt]

Summary

Adds docs/contributor/adr.md — a decision log of RAITAP's durable design choices, wired into the contributor toctree.

Ten records, each with the same fixed structure (Context / Options / Decision / Consequences):

• Adapters self-register via facade decorators
• Deps layer stays torch-free (bootstrap-from-zero)
• Compatibility gates stay separate (revisited 3×)
• Error types only at boundaries
• Non-uniform algorithms use the invoker seam (Expand algorithm registries to full library coverage (torchattacks / foolbox / imagecorruptions) #266)
• Tree backends stamp OutputKind at the forward site
• Label-format adapters raise plain ValueError (Implement pluggable label-format adapters (COCO / YOLO / VOC ingestion) #338)
• Text tokenisation is model-owned (Add text input modality + text-classification use case (end-to-end) #340)
• The tree extra bundles scikit-learn + CPU torch (Add tree/tabular model backend + enable SHAP TreeExplainer end-to-end #246)
• Third-party plugins load at bootstrap, default-allow (Create the 3rd party plugins system via entry-points #173)

Sourced from contributor docs, code comments, and accumulated design notes. Strict Sphinx build (-n -W --keep-going) green.

Checklist

• CI — Required checks are green
• Breaking changes — Not a breaking change; docs only.
• Contributor guide — I've read Pull requests and commit messages before requesting review.

Optional

• Docs (optional) — Adds a contributor ADR page.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an Architecture Decision Records (ADR) page to the contributor documentation, capturing durable design choices and linking it into the contributor docs navigation.

Changes:

• Added docs/contributor/adr.md with ten ADR entries using a consistent Context/Options/Decision/Consequences structure.
• Wired the new ADR page into the contributor docs toctree (docs/contributor/index.md).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/contributor/index.md	Adds adr to the contributor docs toctree so it appears in navigation/build.
docs/contributor/adr.md	Introduces the ADR log page with structured records and cross-cutting principles.