Update Django docs

[jwhitlock]

This refreshes the Django docs, and adds some information that may be useful.

• Add that the DockerflowMiddleware also serves the __version__, __heartbeat__ and __lbheartbeat__ views. In mozilla/fx-private-relay PR #4345, we removed some duplicate versions of the views that were not called because of the middleware.
• Document that check results are logged to dockerflow.checks.register. This is how you can see what checks are failing if DEBUG=False.
• Add an example heartbeat response when DEBUG=False.
• Add dockerflow to the sample logging configuration. Without this (and without a root handler), these logs are discarded, making it impossible to debug heartbeat issues.
• Drop reference to MIDDLEWARE_CLASSES, which was removed in Django 2.0 in 2017.
• Add SECURE_REDIRECT_EXEMPT for Kubernetes deployments.
• Expand the Settings section to document all the added settings and a few Django ones as well:
  • Add Django's setting DEBUG, to document how it affects what checks heartbeat runs, and the data included in the heartbeat response.
  • Add DOCKERFLOW_HEARTBEAT_FAILED_STATUS_CODE, previously mentioned in the heartbeat view section
  • Add DOCKERFLOW_REQUEST_ID_HEADER_NAME, previously mentioned in the "Requests Correlation ID" section. Add why you may want to change it for different deployment environments.
  • Add DOCKERFLOW_SUMMARY_LOG_QUERYSTRING, previously mentioned in the Logging section.
  • Move DOCKERFLOW_VERSION_CALLBACK down to alphabetical order
  • Add Django's setting SILENCED_SYSTEM_CHECKS, used but not previously documented.
  • I did not add BASE_DIR, since there is no Django docs, because Django maintainers say that is a constant, not a setting (https://code.djangoproject.com/ticket/31387).

[jwhitlock]

I decided to use dockerflow instead of dockerflow.checks.register in the sample logging config. This should have the same effect, but allow moving files around or catching logs from other locations. I also fixed the name of RequestIdLogFilter in the example.

[leplatrem]

Thanks @jwhitlock for the contribution 👏

We will have to postpone the review for a few days 🙏

[jwhitlock]

Thanks @leplatrem, I've applied the changes, including RequestIdLogFilter across the project. I've been focused on the Django docs, I see others are better about updating all the docs.

traceparent does look interesting to me. I'd want to see how GCP implements it before making it the default, and maybe talk to the SREs about how we'd integrate it.

[leplatrem]

Thank you!