The /docs/guides/security landing page and regions guide had no mention
of GDPR, ISO 27001, or DDoS protection despite Supabase covering these
in one place or another. Adds a dedicated GDPR compliance guide (data
residency + DPA), quick-win ISO 27001 and DDoS paragraphs to the
landing page, and a data residency section to the regions guide.
Closes DOCS-354.