docs: fix custom model examples

[MukundaKatta]

Summary

• update the custom model guide to match the current custom model loader contract
• document the required abstract properties and remove stale super().__init__(config) / env_config examples
• fix invalid task-string and Python type-hint snippets in the custom model docs

Fixes #760

Validation

• git diff --check
• python3 -m compileall -q src/lighteval/models/custom/custom_model.py
• parsed all Python fenced blocks in docs/source/evaluating-a-custom-model.mdx with ast.parse