Dev Docker setup documents for contributors fix

[Benebo7]

Summary

• Fix invalid placeholder values in surfsense_backend/.env.example (AUTH_TYPE, REGISTRATION_ENABLED) that silently broke some authentication routes
• Expand the "Building from Source (Contributors)" section in the Docker Installation docs with the missing .env files, full service list, and buildx requirement
• Fix the broken DOCKER_SETUP.md link in CONTRIBUTING.md

Closes #1594

Testing

• Ran through the documented flow from docker compose -f docker-compose.dev.yml up --build, and confirmed POST /auth/register returns 201 after the .env.example fix (previously 403 with ambiguous placeholder).

High-level PR Summary

This PR fixes the contributor Docker setup documentation and addresses authentication issues caused by invalid placeholder values. The main changes include correcting the AUTH_TYPE and REGISTRATION_ENABLED values in surfsense_backend/.env.example that were previously breaking authentication routes, significantly expanding the "Building from Source (Contributors)" documentation section with detailed setup instructions including all three required .env files, service port mappings, and buildx requirements, and updating the broken Docker setup link in CONTRIBUTING.md to point to the correct documentation section.

⏱️ Estimated Review Time: 5-15 minutes

💡 Review Order Suggestion

Order	File Path
1	surfsense_backend/.env.example
2	surfsense_web/content/docs/docker-installation/index.mdx
3	CONTRIBUTING.md

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