docs: remove phantom multi-context proof API

[peetzweg]

Removes documentation for a multi-context proof API that does not exist in the bindings.

Both READMEs documented create_multi_context, validate_multi_context, is_valid_multi_context, and the MultiContextResult type as public API. None of these are implemented in packages/verifiablejs/src/lib.rs — the only "multi context" thing in the codebase is a test named test_multi_context_aliases_are_unlinkable, which actually just exercises alias_in_context. A consumer following the docs would hit undefined is not a function.

Removed

• Root README: TOC entry, the "Multi-Context Proofs" concept section, the full API-reference section (all three functions), the orphaned "Encoding Contexts for Multi-Context Functions" helper, and the MultiContextResult interface.
• The @polkadot/types example only demonstrated encoding Vec<Vec<u8>> contexts (multi-context input) — repurposed it to encoding members, which is a real need.
• Package README: the three phantom rows in the API table.
• A create_multi_context mention in the alias_in_context description.

Notes

• Docs-only; no code changes. Patch changeset included.
• Stacked on feat: add encode_members helper + fix README accuracy (#2, #3) #17 to avoid a conflict in the shared package-README API table. Retarget to main once feat: add encode_members helper + fix README accuracy (#2, #3) #17 merges, or merge feat: add encode_members helper + fix README accuracy (#2, #3) #17 first.