docs: move the site references to stratara.tech and retire GitHub Pages

[yesbert]

What this changes

Second slice of the approved change serve-the-docs-from-our-own-server (groups 4–6, 8). stratara.tech has served the documentation since the first deploy run this morning, and docs.stratara.tech is configured on the server as a domain alias with a path-preserving 301 (verified by Host header before any DNS moves).

• Deletes docs.yml and docs/CNAME. GitHub Pages keeps serving its last artifact until the repository setting is unpublished, which is the last step and happens only after the DNS for docs.stratara.tech has moved.
• Re-applies what docs: publish the site at stratara.tech #55 did and Revert "docs: publish the site at stratara.tech" #56 reverted: every live reference now names stratara.tech — README (and its docs badge label), llms.txt, SUPPORT, CONTRIBUTING, both issue templates, both hero-sample READMEs, _appBaseUrl. CHANGELOG.md and the archived change keep the old host; the redirect covers them.
• Rewrites the privacy policy's hosting section: our own server, rented from IONOS SE in Germany, our own logs, deleted by rotation, no transfer outside the EU, no GitHub Pages. Front-matter description updated; the rest of the policy was checked and is unchanged.

How it was verified

• Local gauntlet green (./scripts/local-gauntlet.sh), DocFX with --warningsAsErrors, LandingBadgeTests still pass (no third-party request before consent)
• grep -rn docs.stratara.tech outside openspec/ returns only CHANGELOG.md

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