Add notebook to references (#816)

* Add notebook entry to references

* Link notebook to reference entry mainly because the term was not introduced before

* Add missing spaces

* Changed and extended reference entry

* Change line length

Co-authored-by: Tobias Weiß <[email protected]>

Author: Tobias Weiß

_episodes/02-numpy.md

@@ -82,7 +82,7 @@ a line. These both need to be character strings (or [strings]({{ page.root }}/re
 for short), so we put them in quotes.
 
 Since we haven't told it to do anything else with the function's output,
-the notebook displays it.
+the [notebook]({{ page.root }}/reference/#notebook) displays it.
 In this case,
 that output is the data we just loaded.
 By default,

reference.md

@@ -161,6 +161,11 @@ mutable
 :   Changeable. The value of mutable data can be altered after it has been
     created. See [immutable](#immutable)."
 
+notebook
+:   Interactive computational environment accessed via your web browser, in which you can write 
+    and execute Python code and combine it with explanatory text, mathematics and visualizations.
+    Examples are IPython or Jupyter notebooks.
+
 object
 :   A collection of conceptually related variables ([members](#member)) and
     functions using those variables ([methods](#method)).