Add app- and package-level READMEs with root index and env-var quick reference

[snowfox1003]

Summary

Improve onboarding and day-to-day navigation by adding README.md files at key packages and Django apps, and updating the root README.md to index them. Each app README documents purpose, main run_* / management commands, common tasks, and how to run tests. The core/ tree includes nested READMEs for collectors, operations (GitHub, Slack, markdown, file I/O), and utils.

Motivation

• Contributors had to dig through management/commands/ and docs/ to learn what each app does and which CLI flags matter.
• Root README lacked a single map of “which package has local docs” and a short list of required environment variables.
• core.operations and scheduling (boost_collector_runner, config) are shared infrastructure but were not summarized at the package boundary.

Acceptance criteria

• Root README links resolve to every new package README listed in the index table.
• Each new README accurately reflects current commands and stub vs. implemented behavior (e.g. cppa_user_tracker collector stub called out).
• No secrets or environment-specific values in README content.
• Docs-only change; CI lint/pyright/test jobs unaffected (no Python changes).