docs: add exceptional stages

docs/concepts.md

@@ -26,6 +26,28 @@ returns data as output. The output of the last processing stage is passed to the
 output stage. The output stage is executed only once, and is the last stage in a
 workflow. The output of the output stage is saved to disk.
 
+### Exceptional stages
+
+Of course, not all workflows are as simple as the above. In some cases, you may
+want to checkpoint the process, write out intermediate results, or do some other
+special processing. For this, `fasthep-flow` has the following special stages:
+
+**Provenance Stage**: The provenance stage is a special stagen that typically
+runs outside the workflow. It is used to collect information about the workflow,
+such as the software versions used, the input data, and the output data. The
+provenance stage is executed only once, and is the last stage in a workflow. The
+output of the provenance stage is saved to disk.
+
+**Caching stage**: The caching stage is a special stage that can be used to
+cache the output of a processing stage. The caching stage can be added to any
+processing stage, and will save the output of the processing stage to disk or
+remote storage.
+
+**Monitoring stage**: The monitoring stage is a special stage that can be used
+to monitor the progress of the workflow. The monitoring stage can be added to
+any processing stage, and can either store the information locally or send it in
+intervals to a specified endpoint.
+
 ## Anatomy of an analysis workflow
 
 In the most general terms, an analysis workflow consists of the following parts: