docs: document the "internal API" with reference & howto

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+.. _api-simple-tuto:
+
+Flexible internal API usage
+===========================
+
+The :ref:`internal-api` in slixmpp is used to override behavior or simply
+to override the default, in-memory storage backend with something persistent.
+
+
+We will use the XEP-0231 (Bits of Binary) plugin as an example here to show
+very basic functionality. Its API reference is in the plugin documentation:
+:ref:`api-0231`.
+
+Let us assume we want to keep each bit of binary in a file named with its
+content-id, with all metadata.
+
+First, we have to load the plugin:
+
+.. code-block:: python
+
+    from slixmpp import ClientXMPP
+    xmpp = ClientXMPP(...)
+    xmpp.register_plugin('xep_0231')
+
+This enables the default, in-memory storage.
+
+We have 3 methods to override to provide similar functionality and keep things
+coherent.
+
+Here is a class implementing very basic file storage for BoB:
+
+.. code-block:: python
+
+    from slixmpp.plugins.xep_0231 import BitsOfBinary
+    from os import makedirs, remove
+    from os.path import join, exists
+    import base64
+    import json
+
+    class BobLoader:
+        def __init__(self, directory):
+            makedirs(directory, exist_ok=True)
+            self.dir = directory
+
+        def set_bob(self, jid=None, node=None, ifrom=None, args=None):
+            payload = {
+                'data': base64.b64encode(args['data']).decode(),
+                'type': args['type'],
+                'cid': args['cid'],
+                'max_age': args['max_age']
+            }
+            with open(join(self.dir, args['cid']), 'w') as fd:
+                fd.write(json.dumps(payload))
+
+        def get_bob(self, jid=None, node=None, ifrom=None, args=None):
+            with open(join(self.dir, args), 'r') as fd:
+                payload = json.loads(fd.read())
+            bob = BitsOfBinary()
+            bob['data'] = base64.b64decode(payload['data'])
+            bob['type'] = payload['type']
+            bob['max_age'] = payload['max_age']
+            bob['cid'] = payload['cid']
+            return bob
+
+        def del_bob(self, jid=None, node=None, ifrom=None, args=None):
+            path = join(self.dir, args)
+            if exists(path):
+                remove(path)
+
+Now we need to replace the default handler with ours:
+
+.. code-block:: python
+
+
+    bobhandler = BobLoader('/tmp/bobcache')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.set_bob, 'set_bob')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.get_bob, 'get_bob')
+    xmpp.plugin['xep_0231'].api.register(bobhandler.del_bob, 'del_bob')
+
+
+And that’s it, the BoB storage is now made of JSON files living in a
+directory (``/tmp/bobcache`` here).
+
+
+To check that everything works, you can do the following:
+
+.. code-block:: python
+
+    cid = await xmpp.plugin['xep_0231'].set_bob(b'coucou', 'text/plain')
+    # A new bob file should appear
+    content = await xmpp.plugin['xep_0231'].get_bob(cid=cid)
+    assert content['bob']['data'] == b'coucou'
+
+A file should have been created in that directory.