docs: add docs for validation

docs/source/howtos/2 Validating Scenes.rst

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+..
+   Copyright DB Netz AG and contributors
+   SPDX-License-Identifier: Apache-2.0
+
+===================
+2 Validating Scenes
+===================
+
+Motivation
+##########
+The validation of the project requirements should ideally be done as close to the source of the data as possible. This devkit therefore provides functionality to check for basic errors on the supplier side. If you are unsure whether your applications produce valid annotations, simply use the functions provided here to check. **Only send us data, if the methods below say that no errors are present.** If you find any bugs in the code, just hit us up and we will fix them as soon as possible or create a PR.
+
+Usage
+#####
+For the validation you need 2 things: the scene and the project specific onthology.
+
+The onthology should be provided by your project partner. It is a .yaml-file listing all classes and their attributes.
+
+.. code-block:: python
+
+    from raillabel_providerkit import validate
+    import raillabel
+
+    scene = raillabel.load("path/to/scene.json")
+
+    assert validate(scene, "path/to/onthology.yaml") == []
+
+If this code does not raise any errors, you are good to go. If it does, read the content of the list `validate` returns carefully. It should tell you where the errors are. If you are unsure, contact your project partner or raise an issue on GitHub.