Add tutorial notebooks for waveform analysis workflows (Issue #42)

[Copilot]

Converts example scripts into documented tutorial notebooks under docs/source/tutorials/, providing pedagogical content for catalog access, waveform matching, optimization, and scattering angle calculations.

New Tutorials

• catalog_downloads.ipynb - Unified interface for downloading from SXS, RIT, GRA, ICCUB catalogs
• nr_eob_mismatch.ipynb - Frequency-domain mismatch calculation between NR and EOB waveforms across mass ranges
• optimizing_initial_conditions.ipynb - Dual annealing optimization of EOB E₀/pₚₕ₀ parameters to minimize NR mismatches
• iccub_waveform_integration.ipynb - Fixed Frequency Integration (FFI) method for Ψ₄ → strain conversion
• scattering_angles.ipynb - Hyperbolic encounter trajectory analysis and scattering angle extraction

Documentation Updates

• Added tutorials to docs/source/index.md toctree
• Replaced deprecated jupyter_execute_notebooks with nb_execution_mode in Sphinx config

Each tutorial includes executable examples, parameter explanations, and visualization of results. Original scripts in examples/ retained for reference.

Original prompt

Please start working on issue #42. Use the already existing code under examples as a reference to prepare the pages

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[RoxGamba]

Some of the notebooks do not currently work out of the box; this is due to the changes the library underwent since the initial notebooks/scripts were added.