feat(nodes/mapper): basic introduction

Author: SIMULATAN

docs/nodes/builtin/mapper.md

@@ -0,0 +1,64 @@
+The Mapper Node transforms data from one format to another. It is useful for converting data structures, changing data
+types, or applying custom transformations to the data.
+
+To achieve this, Templating languages are used to define the transformation rules. The Mapper Node supports both Liquid
+and, for simpler use cases, a custom-made templating language that merely supports replacing variables with their
+values.
+
+Since the two engines are totally different, they will be addressed separately.
+
+## Replace
+
+The Batch-inspired syntax allows replacing variable names with their values. The syntax is as follows:
+
+```text
+%variable%
+```
+
+By using this, simple replacements can be made such as:
+
+```text
+This assignment is due on %due_date%.
+```
+
+Accessing nested variables is also possible:
+
+```text
+This assignment is due on %course.assignment.due_date%.
+```
+
+That's it, for all other use cases, Liquid should be used. Therefore, it is mainly useful when your data already has
+the correct data types, the names are simply incompatible.
+
+## Liquid
+
+The Shopify-developed engine is a powerful and flexible templating language that allows for complex data
+transformations. It supports loops, conditionals, filters, and more. \
+The [Liqp](https://github.com/bkiers/Liqp) library provides the Liquid templating engine. Therefore, the behavior may
+not mimic the original Ruby-based Liquid engine perfectly. For the most part, it should work as expected though.
+
+As Liquid is a nicely documented language with a ton of features, please refer to the official documentation:
+https://shopify.github.io/liquid/basics/introduction/
+
+Apart from the generic features, Snoty also provides some custom filters and tags:
+
+1. `unidiff` filter
+    - This filter is used to convert a list of dictionaries into a unified diff format. It takes a list of dictionaries
+      and returns a string representing the unified diff.
+    - Example:
+      ```liquid
+      {% assign diff = diff.change.description.old | unidiff diff.change.description.new %}
+      ```
+
+## Examples
+### List all changes
+````liquid
+## WebUntis-Änderung!
+
+{% for diff in diff.change %}
+- Change: {{ diff[0] }}
+  ```json
+  {{ diff[1] | json }}
+  ```
+{% endfor %}
+````