[Docs] Add dev docs on live documents

docs/dev/01_live_document_types.rst

+Providing live document types from other apps
+=============================================
+
+AlekSIS apps can provide live document types that
+are then managed by Resint. Live document types are
+Django models sub-classing the ``LiveDocument`` model,
+and providing fields and some methods that define how
+the live document is generated.
+
+The following stripped-down example shows how to
+provide a live document type.
+
+.. code-block:: python
+
+   from aleksis.apps.resint.models import LiveDocument
+
+   class AutomaticPlan(LiveDocument):
+       # Template name to render
+       template = "my_app/pdf_template.html"
+
+       # A field to be rendered in the edit form
+       number_of_days = models.PositiveIntegerField(
+           default=1,
+       )
+
+       def get_context_data(self) -> Dict[str, Any]:
+           """Get context data to pass to the PDF template."""
+	   context = {}
+	   # Do something ehre to construct the context data
+	   return context
+
+This basic example generates a PDF by defining an HTML template
+and overriding the ``get_context_data`` method. This method has
+to return a context dictionary, which will then be passed to
+the template.
+
+If you need more control over how the PDF is generated, you
+can instead override the ``update`` methodd:
+
+.. code-block:: python
+
+   class AutomaticPlan(LiveDocument):
+       def update(self, triggered_manually: bool = True):
+           """Re-generate PDF for this live document."""
+
+	   # Do whatever is necessary to get file contents
+
+	   self.current_file.save(self.filename, content)