fix: fail CI when rules.json drifts from the CLI catalogue

[MaximeGaudin]

Summary

• apps/docs/src/data/rules.json is hand-synced from the CLI via pnpm sync:docs, and nothing in CI verified it stayed in sync — a rule edit without a re-sync would publish wrong docs.
• Added a rules-sync CI job: builds the CLI, runs slint rules --json, and diffs the output against the committed rules.json, failing with a pointer to pnpm sync:docs on drift.

Verified both directions locally: the CLI's --json output is currently byte-identical to the committed file (the job passes), and a one-character edit to rules.json is caught by the diff (the job fails with the sync hint).

Fixes #186

Test plan

• In-sync case: cargo run -q -p slint-cli -- rules --json output diffs clean against the committed file.
• Out-of-sync case: simulated drift fails the diff as intended.
• CI green on the PR