Add documentation for docs live mode.

docs/how-to/contribute-docs.rst

@@ -92,6 +92,65 @@ Once your development environment is set up, run:
 The output of the file should be in the ``docs/_build/html`` folder. If there
 are any markup problems, they'll raise an error.
 
+Live documentation preview
+--------------------------
+
+To support rapid editing of documentation, Toga also has a "live preview" mode:
+
+.. tabs::
+
+  .. group-tab:: macOS
+
+    .. code-block:: console
+
+      (venv) $ tox -e docs-live
+
+  .. group-tab:: Linux
+
+    .. code-block:: console
+
+      (venv) $ tox -e docs-live
+
+  .. group-tab:: Windows
+
+    .. code-block:: doscon
+
+      (venv) C:\...>tox -e docs-live
+
+This will build the documentation, start a web server to serve the build documentation,
+and watch the file system for any changes to the documentation source. If a change is
+detected, the documentation will be rebuilt, and any browser viewing the modified page
+will be automatically refreshed.
+
+Live preview mode will only monitor the ``docs`` directory for changes. If you're
+updating the inline documentation associated with Toga source code, you'll need to use
+the ``docs-live-src`` target to build docs:
+
+.. tabs::
+
+  .. group-tab:: macOS
+
+    .. code-block:: console
+
+      (venv) $ tox -e docs-live-src
+
+  .. group-tab:: Linux
+
+    .. code-block:: console
+
+      (venv) $ tox -e docs-live-src
+
+  .. group-tab:: Windows
+
+    .. code-block:: doscon
+
+      (venv) C:\...>tox -e docs-live-src
+
+This behaves the same as ``docs-live``, but will also monitor any changes to the
+``core/src`` folder, reflecting any changes to inline documentation. However, the
+rebuild process takes much longer, so you may not want to use this target unless
+you're actively editing inline documentation.
+
 Documentation linting
 ---------------------