Simplify Docker installation flow and standardize env variables

[initstring]

Motivation

• Make the Docker installation flow more logical by forcing configuration of the env file before starting containers.
• Shorten and clarify authentication guidance so it's clear how to choose between Google SSO and demo mode.
• Warn explicitly that demo mode allows anyone who can reach the sign-in page to log in without an account.
• Standardize environment variable names used by Docker to remove legacy RTAP_* prefixes for clarity.

Description

• Rewrote docs/installation.md to configure the .env first, then choose authentication mode, and finally start Docker, and tightened the auth section with a clear demo-mode warning.
• Updated deploy/docker/.env.example to use unified variable names (AUTH_URL, AUTH_SECRET, INITIAL_ADMIN_EMAIL, ENABLE_DEMO_MODE, POSTGRES_*, PORT, DATABASE_URL, etc.) and adjusted defaults/comments accordingly.
• Updated deploy/docker/docker-compose.yml to reference the new environment variable names and to set a pre-wired DATABASE_URL that points at the postgres container.
• Updated docs/development.md to add repository-root context for commands and an Environment variables section noting the shared env names and TEST_DATABASE_URL for tests.

Testing

• No automated tests were run for these changes.
• The changes are documentation and configuration-only and were validated by local editing and commit.

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Codex Task

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