DOC: added a more comprehensive example cfood for the docs

src/doc/tutorials/example.rst

+Example CFood
+=============
+
+Let's walk through an example cfood that makes use of a simple directory structure. We assume
+the structure to have the following form, starting from the file crawler root:
+
+.. code-block::
+
+   ExperimentalData/
+   
+     2022_TestData/
+     
+       2022-02-17_TestDataset/
+         file1.dat
+         file2.dat
+         ...
+       ...
+     
+     2023_AnotherDataFolder/
+       ...
+       
+     ...
+
+This file structure conforms to the one described in our article "Guidelines for a Standardized Filesystem Layout for Scientific Data" (https://doi.org/10.3390/data5020043). As a simplified example
+we want to write a crawler that creates "Project" and "Measurement" records in CaosDB and set
+some reasonable properties stemming from the file and directory names. Furthermore, we want
+to link the ficticious dat files to the Measurement records.
+
+Let's first clarify the terms we are using:
+
+.. code-block::
+
+   ExperimentalData/            <--- Category level (level 0)
+   
+     2022_TestData/             <--- Project level (level 1)
+     
+       2022-02-17_TestDataset/  <--- Activity / Measurement level (level 2)
+         file1.dat              <--- Files on level 3
+         file2.dat
+         ...
+       ...
+     
+     2023_AnotherDataFolder/    <--- Project level (level 1)
+       ...
+       
+     ...
+
+So we can see, that the three-level folder structure, described in the paper is replicated.
+We are using the term "Activity level" here, instead of the terms used in the article, as
+it can be used in a more general way.
+
+The following yaml cfood is able to match and insert / update the records accordingly:
+
+         
+.. code-block:: yaml
+
+
+  ExperimentalData:            # Converter for the category level
+    type: Directory
+    match: ^ExperimentalData$  # The name of the matched folder is given here!
+     
+     
+    subtree:
+      
+      project_dir:             # Converter for the project level
+        type: Directory
+        match: (?P<date>.*?)_(?P<identifier>.*)
+        
+        records:
+          Project:
+            parents:
+            - Project
+            date: $date
+            identifier: $identifier
+         
+         
+        subtree:
+         
+          measurement:          # Converter for the activity / measurement level
+            type: Directory
+            match: (?P<date>[0-9]{4,4}-[0-9]{2,2}-[0-9]{2,2})(_(?P<identifier>.*))?
+            
+            records:
+              Measurement:
+                date: $date
+                identifier: $identifier
+                project: $Project
+         
+         
+            subtree:
+              
+              datFile:          # Converter for the files
+                type: SimpleFile
+                match: ^(.*)\.dat$  # The file extension is matched using a regular expression.
+                
+                records:
+                  datFileRecord:
+                    role: File
+                    path: $datFile
+                    file: $datFile
+                  Measurement:
+                    output: +$datFileRecord
+
+
+Here, we provide a detailled explanation of the specific parts of the yaml definition:
+
+.. image:: example_crawler.svg
+

src/doc/tutorials/index.rst

 Tutorials
 +++++++++
+
+.. toctree::
+   :maxdepth: 2
+   :caption: Contents:
+   :hidden:
+
+   Example CFood<example>