[docs] Restructured Documentation

Restructured the documentation in a way that allows it to
be included in the Unified Documentation of OpenWISP.

For more information see openwisp/openwisp-docs#107.

docs/developer/admin-utils.rst

@@ -0,0 +1,94 @@
+Admin Utilities
+===============
+
+This section outlines the admin utilities provided by the OpenWISP Users
+module.
+
+.. contents:: **Table of contents**:
+    :depth: 2
+    :local:
+
+``MultitenantAdminMixin``
+-------------------------
+
+**Full python path**:
+``openwisp_users.multitenancy.MultitenantAdminMixin``.
+
+Adding this mixin to a ``ModelAdmin`` class makes it multitenant-capable,
+allowing users to see only items of the organizations they manage or own.
+
+This class has two important attributes:
+
+- ``multitenant_shared_relations``: If the model has relations (e.g.,
+  ``ForeignKey``, ``OneToOne``) to other multitenant models with an
+  ``organization`` field, list those model attributes here as a list of
+  strings. See `how it is used in OpenWISP Controller
+  <https://github.com/openwisp/openwisp-controller/search?q=multitenant_shared_relations>`_
+  for a real-world example.
+- ``multitenant_parent``: If the admin model relies on a parent model with
+  the ``organization`` field, specify the field pointing to the parent
+  here. See `how it is used in OpenWISP Firmware Upgrader
+  <https://github.com/openwisp/openwisp-firmware-upgrader/search?q=multitenant_parent>`_
+  for a real-world example.
+
+``MultitenantOrgFilter``
+------------------------
+
+**Full python path**:
+``openwisp_users.multitenancy.MultitenantOrgFilter``.
+
+This autocomplete admin filter displays only organizations the current
+user can manage. Below is an example of adding the autocomplete
+organization filter in ``BookAdmin``:
+
+.. code-block:: python
+
+    from django.contrib import admin
+    from openwisp_users.multitenancy import MultitenantOrgFilter
+
+
+    class BookAdmin(admin.ModelAdmin):
+        list_filter = [
+            MultitenantOrgFilter,
+        ]
+        # other attributes
+
+``MultitenantRelatedOrgFilter``
+-------------------------------
+
+**Full python path**:
+``openwisp_users.multitenancy.MultitenantRelatedOrgFilter``.
+
+This filter is similar to ``MultitenantOrgFilter`` but displays only
+objects related to organizations the current user can manage. Use this for
+creating filters for related multitenant models.
+
+Consider the following example from `IpAddressAdmin in openwisp-ipam
+<https://github.com/openwisp/openwisp-ipam/blob/956d9d25fc1ac339cb148ec7faf80046cc14be37/openwisp_ipam/admin.py#L216-L227>`_.
+``IpAddressAdmin`` allows filtering `IpAddress objects by ``Subnet``
+belonging to organizations managed by the user.
+
+.. code-block:: python
+
+    from django.contrib import admin
+    from openwisp_users.multitenancy import MultitenantRelatedOrgFilter
+    from swapper import load_model
+
+    Subnet = load_model("openwisp_ipam", "Subnet")
+
+
+    class SubnetFilter(MultitenantRelatedOrgFilter):
+        field_name = "subnet"
+        parameter_name = "subnet_id"
+        title = _("subnet")
+
+
+    @admin.register(IpAddress)
+    class IpAddressAdmin(
+        VersionAdmin,
+        MultitenantAdminMixin,
+        TimeReadonlyAdminMixin,
+        ModelAdmin,
+    ):
+        list_filter = [SubnetFilter]
+        # other options