Docs: link both environments (production + UAT) where readers need them

[DocGerd]

Goal

The repo documentation should link both deployed environments where a reader needs them. Today only CLAUDE.md documents both; the contributor/user-facing docs predate #96 and only know production.

Audit result (develop @ 99725eb):

Doc	Production link	UAT link	Verdict
README.md (line ~25)	✅	❌	Gap — testers have no pointer to try the unreleased develop state
CONTRIBUTING.md (Development section)	❌	❌	Gap — contributors verify unreleased state on a real deployment nowhere; only local npm run dev/e2e is documented
docs/acceptance.md (line ~15)	✅	❌	Optional — the manual acceptance runbook could offer the UAT URL for pre-release passes
CLAUDE.md	✅	✅	Complete (from #96/#106)
SECURITY.md	✅	—	Fine as-is (production scope is correct there)

Tasks

• README.md: add a short UAT pointer next to the live-app link — https://docgerd.github.io/sail_command/uat/ — labeled as the unreleased develop state (unstable, noindex, not the productive version).
• CONTRIBUTING.md: in the Development section, mention the UAT URL as the place to verify unreleased develop state on a real deployment (auto-refreshed on every push to develop).
• (Optional) docs/acceptance.md: note that the runbook can be executed against the UAT URL for pre-release acceptance passes.

Wording caveat for all additions: UAT serves whatever develop currently holds — explicitly not the productive version, may break at any time.

Follow-up to #96. Documentation-only. Not scheduled for the current session — created for a future one.