added how to link function cats guide to doc guide

sphinx-doc/cook_books_md/DataRegularisation.md

@@ -180,7 +180,7 @@ If there are multiple values present within an interval with size according to t
 `freq`, this values get reduced to one single value, that will get assigned to the timestamp associated with the 
 interval.
 
-This reduction depends on the selected :doc:`regularisation <../Functions/regularisation>` method.
+This reduction depends on the selected :doc:`regularisation <../function_cats/regularisation>` method.
 
 For example, [above](#shift), we applied a backwards :py:func:`shift <Functions.saqc.shift>` with a *10* minutes frequency.
 As a result, the first value, encountered after any multiple of *10* minutes, gets shifted backwards to be aligned with

sphinx-doc/how_to_doc_md/HowToDoc.md

@@ -74,6 +74,7 @@ In pycharm one can activate autogeneration of numpy doc style like so:
 * Do not include lines *only* containing two or more `-` signs, except it is the underscore line of the section heading (otherwise resulting html representation could be messed up)
 
 ## hyperlinking docstrings
+
 * Most straight forward way to make documented code content available / linkable, is, adding a rest file containing an
   automodapi directive to the folder `moduleAPIs` - check out the files it already contains as example.
   * adding ``.. automodapi:: foo.bar``, will make the module `foo.bar` and all its content `foo.bar.X` referable by the 
@@ -105,6 +106,19 @@ In pycharm one can activate autogeneration of numpy doc style like so:
 :py:meth: `saqc.SaQC.show` 
 ```   
 
+## Linking to function categories
+
+To link to a group of functions, you might generate a rest landing page for that link and add it to 
+the `function_cats` folder. It already contains the landing pages for the :doc:`generic <../function_cats/generic>` 
+functions and the :doc:`regularisation <../function_cats/regularisation>` functions.
+Those can be linked via relative paths to the `function_cats` folder. From this file, located in `sphinx-doc/how_to_doc`,
+the linking is realized by:
+
+```python
+:doc:`regularisation <../function_cats/regularistaion>`
+:doc:`generic <../function_cats/generic>`
+```
+
 ## Adding Markdown content to the Documentation
 
 - By linking the markdown file "foo/bar.md", or any folder that contains markdown files directly,