Reorganize MLIP base class documentation into dedicated guide

[Copilot]

Addressed review feedback to improve documentation structure for MLIP base classes.

Changes

• Created docs/src/dev-docs/new-mlip.rst: New dedicated guide for adding MLIP-only architectures, moved from inline example in new-architecture.rst
• Simplified docs/src/dev-docs/utils/mlip.rst: Reduced to automodule directive only (lines 1-7)
• Enhanced docstrings in MLIPModel and MLIPTrainer: Moved all documentation content into class docstrings, including:
  • Usage examples
  • List of provided functionality
  • Explanation of when rotational augmentation is needed (when invariance is not enforced in the NN architecture)
• Reverted examples/0-beginner/00-basic-usage.sh: Restored original :ref:tutorials`` reference

The new structure separates the high-level "how to add an MLIP architecture" guide from the API reference, making both easier to find and use.

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