docs: document all contract error codes and simulation response forma…

[extolkom]

Closes #555

Adds a complete reference document for all contract error codes, covering
what each code means and the exact condition in the contract logic that
triggers it. This removes the guesswork developers previously faced when
a transaction reverts with an opaque numeric code.

What's included

• New/updated file: docs/error-codes.md
• All 33 ContractError variants from creator-keys/src/lib.rs (codes 1–33)
• All 9 PollError variants from creator-keys/src/events.rs (codes 20–28)
• Off-chain read-only quote error identifiers from
  creator-keys/src/quote_view_errors.rs
• Each entry documented as: Code | Name | Description | Trigger Condition,
  with citations to the originating function, file, and line number
• A sample Soroban simulateTransaction RPC response showing
  Error(Contract, #16), plus a short TypeScript snippet showing how a
  client can parse the code out of the response

Scope / non-goals

• Documentation only — no contract logic was modified
• Diff is isolated to docs/error-codes.md to minimize merge conflict risk

Acceptance criteria (from #555)

• All error code variants documented
• Each entry includes code, name, description, and trigger condition
• At least one simulation result example included
• Document lives in the contracts repo under docs/

Verification

• cargo fmt --all -- --check — passed
• cargo check --workspace — passed
• Branch rebased on latest origin/main, no conflicts

Notes for reviewers

Please double-check that no error variants were missed, especially any
recently added ones that might not yet be reflected in lib.rs /
events.rs at review time.