Create consolidated configuration reference

[kmcginnes]

Description

Consolidate all environment variable documentation into a single reference page at docs/references/configuration.md with two clear sections:

• Application Configuration — server behavior: host, port, HTTPS settings, certificate paths, logging, proxy settings, CORS
• Default Connection — pre-configuring a database connection: graph type, connection URL, AWS region, service type, IAM auth, and the JSON file approach

Changes:

• Created docs/references/configuration.md combining env vars from development.md and all content from default-connection.md
• Replaced the env var definitions in docs/development.md with a link to the new reference (dev-only variables like GRAPH_EXP_DEV_PORT remain in development.md)
• Removed docs/references/default-connection.md (absorbed into the new page)
• Added the new page to docs/references/README.md
• Updated links in security.md, troubleshooting.md, and deploy-to-ecs-fargate.md

Validation

Documentation-only change. Verify that all links resolve correctly and the new reference page renders properly.

Related Issues

• Fixes Create a consolidated configuration reference #1715
• Part of Update getting started docs #1097

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.