docs(portfolio): add docker-compose example for portfolio + gateway

[ayushsingh82]

Summary

Adds a docker-compose.yaml example wiring together LocalNet, the Wallet Gateway, and the Splice Portfolio dApp, as requested in #1941.

Both the Wallet Gateway and Splice Portfolio already ship official, public Docker images (documented in docs/wallet-gateway/deployment and docs/dapp-building/examples/portfolio), so this composes those two together on top of a LocalNet instance started with the existing pnpm start:localnet tooling, rather than duplicating that (versioned, auto-fetched) LocalNet compose stack.

• examples/portfolio/docker-compose.yaml: wallet-gateway (v1.9.0) + portfolio (v1.9.0) services. wallet-gateway reaches LocalNet's ledger API via host.docker.internal (with a Linux extra_hosts shim); portfolio's config.json is browser-facing so it keeps localhost URLs.
• examples/portfolio/docker/wallet-gateway.config.json: self-signed LocalNet auth config, adapted from the existing checked-in example at docs/dapp-building/examples/json/default-config.json.
• examples/portfolio/docker/portfolio.config.json: matches the strict portfolioConfigSchema in examples/portfolio/src/lib/schemas.ts.
• Fixed the config mount path vs. the existing docs snippet in docs/dapp-building/examples/portfolio/index.md (which mounts a single file over nginx's whole html root) — this mounts to /usr/share/nginx/html/config.json specifically, matching where the portfolio app actually fetches its config (/config.json).
• Linked the new compose example from docs/wallet-gateway/deployment/index.md and examples/portfolio/README.md.

Verification

• docker compose config validates the compose file cleanly.
• Both config JSON files were checked against their respective zod schemas by reading source directly: wallet-gateway.config.json against Config.ts's rawConfigSchema and the self_signed auth fixtures in core/wallet-auth/src/*.test.ts; portfolio.config.json against portfolioConfigSchema in examples/portfolio/src/lib/schemas.ts.
• Not yet live-tested end-to-end against a running LocalNet — Docker daemon wasn't running and disk space was tight in the environment I built this in (LocalNet is a multi-GB multi-container download). Flagging this openly per the LocalNet ledger API port (2975) and self-signed auth shape being taken from the repo's own existing checked-in example config, which gives me reasonable confidence, but a live docker compose up run against a fresh LocalNet would still be good before merge.

Test plan

• Run pnpm script:fetch:localnet && pnpm start:localnet then docker compose up from examples/portfolio/ and confirm the Portfolio UI loads at http://localhost:3333 and can talk to the Wallet Gateway at http://localhost:3030.

Fixes #1941

[ayushsingh82]

@fayi-da whenever you have a chance, would appreciate a look at this one.