docs: add docs on widget plugins

docs/user/plugin_widgets.md

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+(user.plugin_widgets)=
+# PLugin repository widgets
+
+## Adding widgets to the plugin repository
+
+Widgets can be created by users and added to a beamline plugin repository, then they can be used in
+all the same ways as built-in widgets. To make this work, the widget author should follow a few
+simple guidelines.
+
+Widgets should be added in `plugin_repo.bec_widgets.widgets`. They may be added in submodules. If
+so, please make sure that these are properly defined python submodules with `__init__.py` files, so
+that the widgets are discoverable.
+
+### Preparing a widget to be a plugin
+
+- make sure that the widget class inherits from both `BECWidget` as well as `QWidget` or  a subclass
+  of it, such as `QComboBox` or `QLineEdit`.
+- make sure it initialises each of these superclasses in its `__init__()` method, and passes the
+  `parent` keyword argumment on to `QWidget.__init__()`.
+- add `PLUGIN = True` as a class variable to the widget class
+- add `USER_ACCESS = [...]`, including any methods and properties which should be accessible in the
+  client to the list, as strings.
+
+(Search the `bec_widgets` code for one of the above names for examples of these magic variables)
+
+### Example / template
+
+```Python
+class TestWidget(BECWidget, QWidget):
+    USER_ACCESS = ["set_text"]
+    PLUGIN = True
+
+    def __init__(self, parent=None, **kwargs):
+        super().__init__(**kwargs)
+        QWidget.__init__(self, parent=parent)
+        self.setLayout(QHBoxLayout())
+        self._text_widget = QLabel("Test widget text")
+        self.layout().addWidget(self._text_widget)
+
+    def set_text(self, value: str):
+      self._text_widget.setText(value)
+```
+
+### Generating the plugin files and RPC client template
+
+To allow the BEC client to communicate with the GUI server and to know which widgets are available,
+as well as to allow the Qt Designer to find the available widgets, a code generation tool should be
+run to prepare a client file which lists all the available widget classes and functions. Make sure
+you are in the BEC python environment where your plugin repository is also installed, and run:
+
+```bash
+$ bw-generate-cli --target plugin_repo
+```
+
+replacing `plugin_repo` with the name of your repository. This will overwrite the file for
+`plugin_repo.bec_widgets.client`. This file should not be edited by hand, and should always be
+regenerated when changes are made to widgets in the plugin repository. BEC will need to be restarted
+for changes made here to take effect.