feat(doc): Relocate privacy standards survey content (removed from approaches in #151)

[Meyanis95]

The file approaches/approach-privacy-standards-survey.md was removed in the approach v2 migration (#151) because it does not fit the v2 schema: no use_case: anchor, no sub-architectures, no comparison of solutions. It is a standards catalog, not an approach.

The content (7 standards: ERC-3643, ERC-7573, EIP-5564, EIP-6123, EIP-7805, ERC-7945, ERC-8065) is preserved in git history at the parent of the deletion commit.

We should decide where this knowledge lives. Candidates:

• New domains/standards.md (most likely fit)
• New top-level surveys/ directory if more catalogs are coming
• Split per-standard into existing pattern cards' frontmatter

Out of scope for #151.

[zexoverz]

Recovered the survey and put up a PR for it, #187. Quick note on placement since the issue asks where it should live. domains/README says domain pages are thin indexes and they run ~40 lines, so the 294-line catalog would break that convention as domains/standards.md. I went with candidate 2, a surveys/ directory (surveys/privacy-standards.md) plus a one-line pointer from domains/identity-compliance.md. Easy to move to domains/standards.md if you'd rather, and happy to refresh the standard statuses in a follow-up.