Add SSL auto-renewal docs (cadence, force-renewal, DNS-01 troubleshooting)

[lvangool]

Summary

• Adds a new page security/ssl-auto-renewal.mdx covering:
  • Renewal schedule — the scheduler runs Mon/Thu at 14:00 UTC and attempts renewal within 30 days of expiry.
  • Forcing a renewal — the dashboard "update the certificate config" trick to trigger an immediate attempt outside the scheduled window, with a Let's Encrypt rate-limit caveat.
  • DNS-01 / Cloudflare troubleshooting — proxy/token-scope/CNAME-flattening failure modes and how to verify the _acme-challenge TXT record manually.
• Adds cross-links from both "renews every 3 months" Callouts in security/ssl.mdx.

Why

On HS #33515 a customer's certificate lapsed and their site started returning 526, with no docs explaining when auto-renewal runs or how to intervene. This was filed as an umbrella gap (SUP-990) listing six concerns; this PR ships the three highest customer-impact items (cadence, force-renewal, DNS-01 troubleshooting). The remaining three (warning-email design, badge semantics, audit-log coverage) were deliberately scoped out as lower-impact / more internal-process detail.

Linear

• SUP-990 (partial — 3 of 6 items, by design)
• Originating ticket: HS #33515

Test plan

• yarn validate:mdx passes
• New page renders; the renewal-schedule and DNS-01 sections display correctly
• Both cross-links from ssl.mdx resolve to the new page
• Confirm the Mon/Thu 14:00 UTC cadence and 30-day window match current platform behaviour before merge

🤖 Generated with Claude Code

[linear]

SUP-990