Add missing name server content: migration guide, redundancy article, and cleanup

[stephanoodle]

Summary

• New article: Migrating DNS to DNSimple (migrating-dns-to-dnsimple.md) — How-to guide covering the full DNS migration process: record import, verification, name server switch, and propagation monitoring.
• New article: Name Server Redundancy and High Availability (name-server-redundancy.md) — Explanation article covering why multiple name servers matter, Anycast routing, and DNSimple's redundancy architecture.
• Expanded: Pointing a Domain to DNSimple (pointing-domain-to-dnsimple.md) — Rewritten from a thin routing page into a proper article with delegation context, name server list, propagation expectations, and troubleshooting links.
• Retired: Troubleshooting DNSimple Name Servers (troubleshoot-dnsimple-name-servers.md) — Deleted and redirected to /articles/troubleshooting-name-servers/. Updated all internal cross-links (4 articles) to point to the new URL.
• Updated categories/name_servers.yaml with the two new articles.

Dependencies

This PR should be merged after PR #1746, which creates the redirect target (troubleshooting-name-servers.md).

Test plan

• Verify new articles render correctly with TOC, callouts, and cross-links
• Confirm redirect from /articles/troubleshoot-dnsimple-name-servers/ resolves to /articles/troubleshooting-name-servers/
• Check that updated cross-links in dns.md, check-resolution-status.md, resolution-status.md, and domain-resolution-issues.md point to valid targets
• Verify category page lists the new articles under Explanation and How to sections

[stephanoodle]

Merge order

This PR must merge after #1746, which creates troubleshooting-name-servers.md — the redirect target for the deleted troubleshoot-dnsimple-name-servers.md.

Also note: both this PR and #1744 modify pointing-domain-to-dnsimple.md, so whichever merges second will need a merge conflict resolved on that file.