Add documentation for debugging tools

docs/developers/debugging.rst

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+Debugging Funkwhale
+===================
+
+In order to track down errors its useful to provide as many information as possible. Usually pasting
+the logs should be sufficient, but there are some tools for some deeper debugging.
+
+Frontend Logs
+-------------
+
+Logs and errors written by the Frontend can be accessed with Firefox. When opening the website of
+your Funkwhale instance, simply hit ``Ctlr + Shift + J``. Alternatively open the Firefox Menu and open
+the Browser Console in the developers menu.
+
+In the opening window you can see all the output. You can copy what you want to share or repeat the
+failing operation to see what error occurs.
+
+Backend Logs
+------------
+
+Depending on your setup you can see the logs from our API server in different ways.
+
+Docker
+^^^^^^
+
+Simply run ``docker-compose logs --tail=100 api`` If you want continuous logs, add the ``f`` flag.
+
+Quick install
+^^^^^^^^^^^^^
+
+To get the logs, run ``journalctl -xn -u funkwhale-server``
+
+Profiling
+---------
+
+In order to find performance issues, its possible to run API requests with activated profiling. In
+order to do this,  add ``funkwhale_api.common.middleware.ProfilerMiddleware`` to the environment
+variable ``ADDITIONAL_MIDDLEWARES_BEFORE``
+
+If enabled, simply add ``?prof`` to the request URL you want to profile. You should get an HTML-Report
+of the running request.
+
+Memory Tracing
+--------------
+
+Its possible to print memory traces for each API request to the API logs. In order to do this, add
+``funkwhale_api.common.middleware.PymallocMiddleware`` to the environment variable
+``ADDITIONAL_MODDLEWARES_BEFORE`` This adds a middleware which should not do anything by default.
+Tracing can be activated by setting ``PYTHONTRACEMALLOC=1`` This might has some inpact on the
+performance, please report how it goes. The Middleware now prints the top 25 memory allocations to
+the API logs.