Add missing API reference docs for ~30 undocumented functions

[brendancol]

Author of Proposal: xarray-spatial contributors

Reason or Problem

About 30 public functions exported from xrspatial/__init__.py have no entries in the API reference docs under docs/source/reference/. Several module categories (hydrology, terrain metrics, interpolation) have no .rst files at all, and classification.rst only covers 4 of its 10 exported functions.

Proposal

Design:

Add missing entries to existing .rst files and create new .rst pages for undocumented categories:

1. classification.rst -- add binary, box_plot, head_tail_breaks, maximum_breaks, percentiles, std_mean
2. hydrology.rst (new) -- flow_accumulation, flow_direction, flow_direction_dinf, flow_length, flow_path, sink, fill, hand, basin, basins, watershed, snap_pour_point, stream_link, stream_order, twi
3. terrain_metrics.rst (new) -- roughness, tpi, tri
4. interpolation.rst (new) -- idw, kriging, spline
5. proximity.rst -- add surface_distance, surface_allocation, surface_direction
6. surface.rst -- add erode
7. utilities.rst (new) -- mahalanobis, emerging_hotspots, polygonize, diagnose
8. index.rst -- add new pages to the toctree

Usage:

After this change, every public function in the package has an API reference entry.

Value:

Gets API reference coverage from ~60% to 100% of exported functions.

Stakeholders and Impacts

Documentation only. No code changes.

Drawbacks

More .rst files to maintain, but they mirror the source modules and follow a mechanical pattern.

Alternatives

Could use a single autodoc directive against __init__.py, but explicit .rst files give better control over grouping.

Unresolved Questions

None.