feat: add figure combination utilities (overlay, add_secondary_y, update_traces) (#16)

* Files created/modified:
  ┌───────────────────────────────┬───────────────────────────────────────────────────────────────────────────────┐
  │             File              │                                    Changes                                    │
  ├───────────────────────────────┼───────────────────────────────────────────────────────────────────────────────┤
  │ xarray_plotly/figures.py      │ Renamed combine_figures → overlay_figures (with alias), added add_secondary_y │
  ├───────────────────────────────┼───────────────────────────────────────────────────────────────────────────────┤
  │ xarray_plotly/__init__.py     │ Exported overlay_figures, add_secondary_y, combine_figures                    │
  ├───────────────────────────────┼───────────────────────────────────────────────────────────────────────────────┤
  │ tests/test_figures.py         │ Added 16 new tests for add_secondary_y and alias verification (34 total)      │
  ├───────────────────────────────┼───────────────────────────────────────────────────────────────────────────────┤
  │ docs/examples/combining.ipynb │ New notebook demonstrating both methods                                       │
  └───────────────────────────────┴───────────────────────────────────────────────────────────────────────────────┘
  API:

  from xarray_plotly import overlay_figures, add_secondary_y

  # Overlay traces on same axes
  combined = overlay_figures(area_fig, line_fig)

  # Dual y-axis (different scales)
  combined = add_secondary_y(temp_fig, precip_fig, secondary_y_title="Rain (mm)")

  Features:
  - overlay_figures: Supports facets, animation, multiple overlays
  - add_secondary_y: Supports animation, custom y-axis title
  - Both create deep copies (originals not modified)
  - Both validate compatibility and raise clear errors

  Test results: 99 tests passing

* Update notebook

* Update notebook

* 1. Added facet support to add_secondary_y - The function now creates secondary y-axes for each facet subplot (e.g., y→y4, y2→y5, y3→y6)
  2. Updated tests - Added 6 new tests for faceted secondary y-axis:
    - test_matching_facets_works
    - test_facets_creates_multiple_secondary_axes
    - test_secondary_traces_remapped_to_correct_axes
    - test_mismatched_facets_raises
    - test_mismatched_facets_reversed_raises
    - test_facets_with_custom_title
  3. Updated notebook (docs/examples/combining.ipynb):
    - Added new "With Facets" section showing add_secondary_y working with faceted figures
    - Changed "No Facet Support" limitation to "Mismatched Facet Structure" showing the error when structures don't match
    - Updated summary table: add_secondary_y now shows "Yes (must match)" for Facets
  4. All 103 tests pass

* Add notebook showing off manipulation options

* Updated the notebook to:

  1. Use overlay_figures for adding traces - shown under "Easy: Adding traces to faceted/animated figures"
  2. Keep just two helpers:
    - update_animation_traces() - the main pain point
    - set_animation_speed() - for the deeply nested API
  3. Added facets + animation example showing the helper works for both

  So the final picture is:
  ┌─────────────────────────────────────────────────────┬──────────────────────────────────────────┐
  │                 What you want to do                 │                 Solution                 │
  ├─────────────────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Add traces to animated/faceted figures              │ overlay_figures() ✅ already in library  │
  ├─────────────────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Update trace style (line_width, etc.) on animations │ update_animation_traces() - needs helper │
  ├─────────────────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Change animation speed                              │ set_animation_speed() - needs helper     │
  ├─────────────────────────────────────────────────────┼──────────────────────────────────────────┤
  │ Everything else                                     │ Works out of the box                     │
  └─────────────────────────────────────────────────────┴──────────────────────────────────────────┘
  Should we add update_animation_traces() and set_animation_speed() to xarray_plotly.figures as proper exported functions? They're simple but solve real pain points.

* Final Public API

  from xarray_plotly import (
      xpx,                      # Main entry point - accessor with IDE completion
      overlay,                  # Combine figures on same axes
      add_secondary_y,          # Dual y-axis plots
      update_animation_traces,  # Update traces in animation frames
      config,                   # Configuration settings
  )

  Changes Summary
  ┌─────────────────────────┬──────────────────────────────────┐
  │         Removed         │              Reason              │
  ├─────────────────────────┼──────────────────────────────────┤
  │ overlay_figures         │ Renamed to overlay               │
  ├─────────────────────────┼──────────────────────────────────┤
  │ combine_figures         │ Alias removed                    │
  ├─────────────────────────┼──────────────────────────────────┤
  │ SLOT_ORDERS             │ Implementation detail            │
  ├─────────────────────────┼──────────────────────────────────┤
  │ DataArrayPlotlyAccessor │ Users use xpx()                  │
  ├─────────────────────────┼──────────────────────────────────┤
  │ DatasetPlotlyAccessor   │ Users use xpx()                  │
  ├─────────────────────────┼──────────────────────────────────┤
  │ auto                    │ Rarely needed                    │
  ├─────────────────────────┼──────────────────────────────────┤
  │ set_animation_speed     │ Kept as local helper in notebook │
  └─────────────────────────┴──────────────────────────────────┘
  ┌─────────────────────────┬─────────────────────────┐
  │          Added          │         Reason          │
  ├─────────────────────────┼─────────────────────────┤
  │ update_animation_traces │ Was hidden, now exposed │
  └─────────────────────────┴─────────────────────────┘

* Rename update_animation_traces to update_traces

* Make module callable

* Revert

Author: FBumann