Validation Framework: codeowner and contributor documentation

[hdamker]

Problem description

The Validation Framework v1 (#448) is implemented and entering RC
phase. As it becomes part of every CAMARA API repository's PR and
release flow, two groups of readers need documentation that today does
not exist or is scattered across design documents, code comments, and
PR descriptions: codeowners of API repositories who encounter the
validation day-to-day, and contributors who change the validation
itself.

The most important deliverable is the codeowner-facing material,
because that audience is the broadest and the validation only works if
codeowners can interpret and act on findings without help.

Possible evolution

• Codeowner documentation — what the validation checks at PR time
  vs. release time, how to read a validation result, and a growing FAQ
  for specific rule findings that recur in real repositories.
• GitHub App landing page in camaraproject/tooling — the entry
  point for codeowners who first see the CAMARA Validation app on
  their PRs and want to know what it is, what permissions it has, and
  where the findings come from.
• Contributor / extensibility guide — how to add or modify a
  Python check or a Spectral rule, how the engines, post-filter, and
  output pipeline interact, written for human contributors and AI
  agents working on the codebase.
• Architecture overview — a single document that ties together the
  design documents in ReleaseManagement (docs: add validation framework requirements and detailed design #447, bundling design) with
  the implementation in camaraproject/tooling, so a new reader can
  navigate from "what does this thing do" to "where does the code
  live".

Initial scope

Documents live in camaraproject/tooling under validation/docs/,
alongside the existing regression-testing manual (#483 first delivery).
The documentation targets the steady state where the validation
code lives on main (after validation-framework has been merged),
not the current pre-merge branch layout. Markdown only; cross-linked
from a top-level validation/docs/README.md index and from the
tooling repository's top-level README.md.

Dependencies / relationship to other work

• Implement CAMARA Validation Framework v1 #448 (Validation Framework v1 implementation) — implementation
  must be stable enough that documentation does not constantly drift
• Validation Framework: regression testing infrastructure #483 (regression testing infrastructure) — the regression-testing
  manual sets the precedent for validation/docs/ layout

Additional context

CAMARA-wide guidance on release process and codeowner responsibilities
belongs in ReleaseManagement or project-administration, not in this
issue's scope. The documentation tracked here is validation-specific:
"how do I read and act on a Validation Framework finding", and "how
do I extend the validation".