docs: move the documentation site to implementation.opensysml.org

[devin-ai-integration]

What and why

The opensysml.org apex domain becomes the OpenSysML ecosystem landing page, published from Open-MBEE/opensysml.github.io (Open-MBEE/opensysml.github.io#1). The documentation site moves to implementation.opensysml.org:

• mkdocs.yml site_url → https://implementation.opensysml.org/ (canonical links, sitemap)
• New docs/CNAME → implementation.opensysml.org, so the Pages artifact claims the subdomain on deploy and releases the apex
• Every link to the published site repointed: README badge and docs link, CONTRIBUTING, the three issue templates, the nightly release notes message, spec-compliance.md, and the header menu, which also gains a link back to opensysml.org

SysML model URIs like attribute redefines target = "https://opensysml.org/" in the manual examples are model data, not site links, and are unchanged.

How it was verified

• python -m mkdocs build --strict passes; site/CNAME and the canonical URL in site/index.html confirm the new domain
• python3 scripts/check-doc-links.py and python3 scripts/changelog.py check pass

Checklist

• make test and make lint pass locally (docs-only change: mkdocs build --strict, link and changelog checks run instead)
• Tests added or updated for the change (n/a — site configuration)
• Documentation extended where it already covers the surface (see CONTRIBUTING.md)
• Changelog entry added as changes/unreleased/<slug>.<section>.md, not as an edit to CHANGELOG.md
• baselines regenerated and make docs-counts run if a gate count moved (no gate counts moved)
• No internal work-item labels (waves, slices, F4, K5) in the body, docs, or changelog

Link to Devin session: https://nasa-jpl-demo.devinenterprise.com/sessions/39fe31b6ac564016859146b2f1761a6a
Open in Devin Desktop: https://nasa-jpl-demo.devinenterprise.com/desktop/session/39fe31b6ac564016859146b2f1761a6a?variant=devin
Requested by: @HuiJun

[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

• Disable automatic comment, CI, and merge conflict monitoring

[devin-ai-integration]

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[devin-ai-integration]

Note on the failing Go race tests (rest) check: tests/migrate fails on TestInstantBehaviorsWaitingForTheClockAreLedgered and TestGoldenConstructFixtures/instant_waits, both against testdata/xmi/instant_waits.golden.sysml. The same failures reproduce on a clean checkout of origin/develop (9365f43), so this is a preexisting develop issue unrelated to this docs-only diff — fix/instant-waits-golden is already in flight upstream. Build and test fails only as the aggregate gate over that check. I'll rebase onto develop once the golden fix lands.