docs(domains): add sslip.io domain details and usage guidelines

[Cinzya]

Summary

• Add a dedicated "sslip.io Domains" section to docs/knowledge-base/domains.md explaining what sslip.io is, when Coolify falls back to it (no Wildcard Domain set on the server), and what it's good for.
• Call out the HTTPS gotcha inline: Let's Encrypt rate-limits the entire shared sslip.io zone, so auto-generated sslip URLs stay on http:// — switching them to https:// will fail certificate issuance. This matches Coolify's in-app warning (warning.sslipdomain) and the sslipDomainWarning() helper in bootstrap/helpers/shared.php.
• Note in the DNS Validation section that *.sslip.io hostnames are automatically skipped during DNS validation, matching the early-return in validateDNSEntry().

Test plan

• bun install && bun run dev, then open http://localhost:5173/docs/knowledge-base/domains and confirm the new "sslip.io Domains" section renders between the intro and "HTTPS & SSL Certificates".
• Confirm the info and tip callouts render with the correct styles in both light and dark mode.
• Click the in-page anchor #wildcard-domain and verify it scrolls to the existing Wildcard Domain section.
• Click the external sslip.io and letsencrypt.org links and verify they carry the ?utm_source=coolify.io query param.
• bun run build succeeds with no new VitePress errors (pre-existing unresolved-link warnings on lines 59/98/102/105/110 are unrelated to this change).