Docs: fix .env.example paths in external services guide

[yuanchima]

Problem

The external services guide currently references these example env files without the leading dot:

• apps/api/env.example
• apps/worker/env.example

In the repository, the actual files are:

• apps/api/.env.example
• apps/worker/.env.example

This can trip up new contributors following the minimum local setup path from docs/external-services.md.

Suggested fix

Update the two references in docs/external-services.md to include the leading dot, matching the README Quick Start and the actual files in the repo.

[suguanYang]

Misleading docs were removed in #55