docs: document the estimator config API

[satwiksps]

What does this PR do?

This is the documentation PR for the neural network API refactor. Currently work in progress and a draft untill PR9 of GSoC lands

Does this close any issues?

Anything else we should know?

Checklist

• I have read the contributing guide.
• uv run pytest -n auto -m "not slow and not gpu" passes.
• uv run pre-commit run --all-files passes (ruff and formatting).
• uv run pyright sbi passes.
• I added or updated tests for the changed behavior.
• I used Google-style docstrings for new or changed public functions.
• (If applicable) I reported how long new tests run and marked slow ones
  with pytest.mark.slow.

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