Document the synthetic analyzer fixture contribution workflow

[achieve0410]

Problem

The repository contains deterministic React, Vue, Nuxt, and Django fixtures, but a new contributor currently has to infer the fixture structure and focused verification commands from multiple files.

Scope

Add a concise contributor guide for creating or extending a synthetic analyzer fixture. This is documentation-only and must not change analyzer, graph, API, runtime, storage, or network behavior.

Acceptance criteria

• add a guide under docs/ describing the existing fixture layout and when to extend an existing fixture versus add a new one
• require synthetic, non-sensitive data and prohibit credentials, private source, absolute local roots, request/response bodies, and analyzed-project data
• explain which focused JavaScript, Python, and Playwright checks apply to fixture changes
• link docs/graph-contract.md and docs/security-model.md where contract or security boundaries are relevant
• link the new guide from CONTRIBUTING.md
• keep all commands consistent with the current package scripts and ./scripts/check.sh

Verification

• verify every referenced file and command exists
• run documentation-relevant static checks and state any intentionally unrun full check