Add documentation to datagov-catalog README

[SueValente]

What

Submit a PR to add documentation to the datagov-catalog README. A full draft of the updated README has been prepared.

Why

The current README only covers local development setup. It is missing architecture overview, deployment instructions, environment variables documentation, and links to related resources. This makes it harder for new team members to understand and operate the application.

Done When

• README updated with About section describing the application and its role in the platform
• Architecture section added covering Flask app, Postgres DB, OpenSearch, S3, New Relic, and Logstack
• Local development setup expanded with prerequisites and full setup steps
• Environment variables table added based on .env.sample
• Deployment section added referencing the deploy workflow
• Related resources section added linking to wiki and harvester repo

Notes

Draft content is available as a comment on epic #5832. The deployment section has been verified against the actual deploy workflow. The only item needing verification before merging is whether logstack is still actively in use.