Add coding conventions

CONVENTIONS.md

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+# Coding Convetions
+
+- Write pragmatic, easily testable, and performant code!
+- Prefer short and pure functions where possible!
+- Keep the number of function arguments as low as possible!
+- Don´t use nested functions!
+- Write concise and to-the-point docstrings for all functions!
+- Use type comments style (PEP 484) instead of function annotations!
+- Always add a correct PEP 484 style type comment as the first line after the function definition!
+- Use built-in collection types as generic types for annotations (PEP 585)!
+- Use the | (pipe) operator for writing union types (PEP 604)!
+
+Example function with type annotations and docstring:
+
+```python
+def tokenize_chunks(chunks, max_len=None):
+    # type: (list[str], int|None) -> dict
+    """
+    Tokenize text chunks into model-compatible formats.
+
+    :param chunks: Text chunks to tokenize.
+    :param max_len: Truncates chunks above max_len characters
+    :return: Dictionary of tokenized data including input IDs, attention masks, and type IDs.
+    """
+```