docs: add organization profile and contribution guide

[kerberizer]

Motivation

The organization currently presents generated package repositories, production infrastructure, staging work, unfinished security and mirror projects, and historical Buildbot code without an organization-level map. Contributors can easily choose a generated repository instead of the controlling source, while users have little context for the current experimental release name.

Changes

• Add the public ArchZFS organization profile with user entry points and repository classifications.
• Add a concise inherited CONTRIBUTING.md that routes changes to the controlling repository.
• Add repository-local agent instructions and a Claude import for maintaining organization-wide documentation safely.
• Explain how the current public release tracks the configured released OpenZFS version and current Arch kernels, including per-kernel fallback when a newer kernel cannot be built.

The profile labels keyring packaging and reusable mirror tooling as unfinished and archzfs-ci as historical rather than presenting them as deployed alternatives.

Organization-wide impact

CONTRIBUTING.md becomes GitHub’s default contribution guide for organization repositories that do not provide a local override, including generated package repositories. GitHub displays this inherited file but does not copy it into repository clones. Repository-critical operational rules therefore remain in each controlling repository’s own instructions.

No security, governance, support, issue-template, or PR-template defaults are added; those require separate policy decisions.

Validation

• Ran git diff --check across the complete branch diff.
• Verified the organization profile and default community-file locations against GitHub documentation.
• Checked repository roles and links against current organization metadata, release assets, workflows, and open keyring/mirror work.
• Reviewed the Markdown structure and the organization-wide inheritance boundary.

Documentation only. This does not change packages, workflows, releases, signing, repository settings, or generated sources.

Coordination

The current-architecture and agent guidance is under review in archzfs/archzfs#644. Wiki refresh commits 46235a0 and fd5baee are published. Temporary dependency notices remain on the Wiki Home and Development pages until this PR and archzfs/archzfs#644 merge.

Wiki follow-up

• After this PR merges, remove the temporary docs: add organization profile and contribution guide #1 dependency notices from the Wiki Home and Development pages and verify the contribution-guide links.

[kerberizer]

An important point about the review: I'm mostly looking for an approval on the general direction and only possibly -- if you can find the necessary time, which very understandably can be quite tight -- finding areas for improvement or necessary corrections. In other words, I absolutely don't want to burden you with tons of content to review.

I have myself reviewed as carefully as possible the PR changes, but it's still just a single pair of eyes and a single, inevitably limited, human brain. :)