docs: formalize documentation and normative language rules

[alexgwolff]

Summary

Establishes a consistent documentation workflow for Kraf's contributors and agents, with explicit normative language, traceable specifications, and navigation through documentation links. Repository-wide documentation is organized under docs/, while the root keeps the repository README and agent entry point.

What does this pull request change?

• Formalizes RFC 2119 and RFC 8174 normative language, Markdown front-matter, documentation navigation, and language-neutral documentation rules.
• Adds docs/ARCHITECTURE.md for Kraf's target architecture, consolidates repository conventions in docs/BUILDING.md, and separates code invariants into docs/CODE_STANDARDS.md.
• Adds docs/README.md as the documentation index and updates links and the allowlist for the new structure.
• Moves specification conventions to docs/SPEC_CONVENTIONS.md, aligns intent filename conventions, and defines ADR structure, statuses, and supersession links while preserving accepted decision content.
• Replaces the docs-review skill with a docs skill for creation, editing, and review, including scoped inventories, document reachability, and link/anchor checks.
• Makes AGENTS.md the documentation entry point and requires agents to follow relevant links; searches remain available for inventory and validation.
• Updates related-document navigation, configuration-file links, illustrative examples, and the MacOS spelling in the environment-variable host acceptance document.

How to test?

• Review the documentation against docs/BUILDING.md and docs/SPEC_CONVENTIONS.md, using the docs skill.
• Navigate from the repository README and AGENTS.md through docs/README.md; verify document reachability, relative-link targets, and identifier anchors.
• Check that ADR supersession updates only the old ADR's status, linking to the successor, while the new ADR links to the predecessor in Links.
• The last documentation review verified 62 relative links and their anchors, document reachability, and the example intent's link to the index. The two remaining bare file references were then converted to links and read back after committing. Environment-variable feature docs were excluded from the review. No build or runtime tests were run.

Other Notes and Links

• docs/ARCHITECTURE.md describes the target architecture, including planned capabilities.
• Feature documentation and the illustrative examples remain in their existing folders.
• Normative terminology: RFC 2119 and RFC 8174.