docs: add log file documentation

No API changes

Closes #225

docs/logging.md

+## Inject logging structure
+
+The logs for this project are stored on the path specified in `INJECT_LOGS`,
+which is set to `backend-logs.jsonl` by default.
+
+### Format
+
+The log file contains messages in the JSONL [format](https://jsonlines.org/).
+Each message contains these fields:
+- `module` - module where this message originates from
+- `level` - log level, can be either `INFO`, `WARNING` or `ERROR`
+- `date` - timestamp when the message was created
+- `lineno` - line number
+- `message` - the message text
+
+### Troubleshooting
+
+The logs contain quite a few messages and most of them won't be useful to admins for
+figuring out why the platform does not behave as expected.
+This is because the exercise is quite a complex state machine,
+therefore we log all actions that happen during the exercise to help developers replicate bugs.
+
+However, despite this _all_ `WARNING` and `ERROR` level logs should be considered when checking logs.
+These logs might hint at the possible reasons why something does not work.
+
+### Troubleshooting tips
+
+- When debugging the process of sending emails with credentials,
+  you can search for the word `credentials` in the `message` field.
+  This should return _all_ logs related to sending these emails.