Configure method documentation

Closes #110 (closed)

docs/contents/implem/configure.md

+# Configuration
+
+When the worker is running over elements, be it locally or on Arkindex, the first step before actually doing
+anything is configuration. This process is implemented in the `configure` method that a worker
+inherits from [ElementsWorker][elements_worker] and [BaseWorker][base_worker].
+This method can also be overloaded if the worker needs additional configuration steps.
+
+The developer mode was designed to help worker developers reproduce and test how their worker
+would behave on Arkindex. This is why the configuration process in this mode mirrors the operation while
+replacing API calls by CLI arguments.
+
+The developer mode is enabled when at least one of three events occur:
+- the `--dev` CLI arguments is used,
+- the `WORKER_VERSION_ID` variable was not set in the environment,
+- the `ARKINDEX_WORKER_RUN` variable was not set in the environment.
+
+None of these happen when running on Arkindex.
+
+
+## Developer mode
+- The worker's configuration YAML with variable needed by the worker
+contains also the list of secrets needed by the worker. See [secrets][].
+
+- ARKINDEX_CORPUS_ID to specify which corpus the processed elements belong to
+- Local secrets loading
+
+- DEBUG mode
+When implementing a new worker, some additional logs might be needed to properly investigate
+why something is not working as intended. The logging level can be set to the `DEBUG` level via either
+- the `--verbose` CLI arguments,
+- setting the `ARKINDEX_DEBUG` to `True` in the environment,
+- specifying the `"debug": True` in the worker's configuration via the `user_configuration`.
+    For more information, see [how to use the user_configuration][user-config].
+
+
+## Arkindex mode
+
+- DEBUG mode
+- RetrieveWorkerRun, what is a worker run. link to arkindex api ? what information does it give
+- user_configuration loading
+ reading default values and storing them in the config
+- secrets actual loading
+- overriding the config with worker's configuration
+
+
+[elements_worker]: /../../../ref/elements_worker#elements-worker
+[base_worker]: /../../ref/base_worker#base-worker
+[user-config]: /../workers/yaml.md#setting-up-user-configurable-parameters