Apply Diataxis structure to developer's guide

Categorize contents in developer's guide as how-to guides and reference
documentation.

Merged-by: Martin Grzenia <martin.grzenia@iml.fraunhofer.de>

Author: swltr

opentcs-documentation/src/docs/developers-guide/01_general-development.adoc

@@ -0,0 +1,6 @@
+// SPDX-FileCopyrightText: The openTCS Authors
+// SPDX-License-Identifier: CC-BY-4.0
+
+== Tutorials
+
+TBD