docs(plugin): explain plugin registration

In the legacy branch, the plugin documentation described plugins get
discovered by being in the same environment. That documentation was not
ported to the new branch. The doc was originally added by
hff7143e7c991b1a80e7ec1ea6836ef3a21b5a812

I also wondered how the discoverability works via Pluggy, that is done
by having the plugin to register a `tox` entry-point which allows tox to
find the plugin module. Document that.

While writing a plugin, that would have helped me find out how to
install or enable it when it is indeed automatic (as long as a tox
entry-point is defined by the plugin).

docs/changelog/3116.doc.rst

@@ -0,0 +1 @@
+Explain how plugins are registered and discovered - by :user:`hashar`.

src/tox/plugin/__init__.py

@@ -1,6 +1,19 @@
 """
-tox uses `pluggy <https://pluggy.readthedocs.io/en/stable/>`_ to customize the default behaviour. For example the
-following code snippet would define a new ``--magic`` command line interface flag the user can specify:
+tox uses `pluggy <https://pluggy.readthedocs.io/en/stable/>`_ to customize the default behaviour. It provides an
+extension mechanism for plugin management an calling hooks.
+
+Pluggy discovers a plugin by looking up for entry-points named ``tox``, for example in a pyproject.toml:
+
+.. code-block:: toml
+
+    [project.entry-points.tox]
+    your_plugin = "your_plugin.hooks"
+
+Therefore, to start using a plugin, you solely need to install it in the same environment tox is running in and it will
+be discovered via the defined entry-point (in the example above, tox will load ``your_plugin.hooks``).
+
+A plugin is created by implementing extension points in the form of hooks. For example the following code snippet would
+define a new ``--magic`` command line interface flag the user can specify:
 
 .. code-block:: python