docs(contributing): welcome all contribution shapes

[huangruiteng]

What changed

Rewrite the opening of CONTRIBUTING.md with a warmer, hyper-inspired welcome that names the many non-code ways to contribute (filing/triaging issues, docs and examples, answering questions, PR review) before diving into task claiming and validation rules. Also drops the stale "project is early" wording that README already removed.

Why

The previous opening ("contributions are small, reviewable, and tied to a public task or clear bug") read as a gate rather than an invitation. Lowering the first-contact friction makes the contributor board and review flow easier to approach, without changing any engineering requirements below.

Validation

• git diff --check clean (1 file, +16/-2)
• loopx check --scan-path CONTRIBUTING.md: public boundary scan clean
• loopx canary premerge --from-git-diff: merge gate passed, self-merge allowed, no manual holds

[huangruiteng]

Self-merge decision (maintainer; docs-only)

Changed surface: CONTRIBUTING.md opening section only — a warmer,
hyper-inspired welcome that names non-code contribution shapes, plus removal
of the stale "project is early" wording. No engineering requirements changed.

Checks run:

• git diff --check: clean (1 file, +16/-2).
• loopx check --scan-path CONTRIBUTING.md: public boundary scan clean.
• loopx canary premerge --from-git-diff: merge gate passed,
  self_merge_allowed=true, no manual holds.
• GitHub CI: DCO Sign-off pass, dependency-review pass; no other checks
  triggered for a docs-only change.

Manual holds: none.

Why this coverage is enough: the diff is a single-purpose contributor
metadata change with no runtime, benchmark, permission, or evidence-policy
impact; the focused boundary scan and diff hygiene are the applicable
validation for this surface.