docs: migrate userguide, examples, and notebooks to the Water Data API

[thodson-usgs]

Migrates the documentation and demo notebooks off the deprecated nwis module onto the waterdata module (USGS Water Data API).

Changes

• userguide/timeconventions — rewritten for the Water Data API datetime model: time is a column (tz-aware UTC for continuous data, tz-naive for daily), with the .dt.tz_convert idiom.
• examples/readme_examples + siteinfo_examples — ported to get_continuous / get_monitoring_locations / get_time_series_metadata.
• demo notebooks — migrated to the waterdata API (USGS-prefixed monitoring-location ids; get_daily, get_continuous, get_field_measurements, get_peaks, get_ratings, get_stats_*, get_samples), with titles, narratives, and argument lists rewritten to match the code. WaterUse stays on legacy nwis.get_water_use with a note (no Water Data API equivalent yet).
• renamed the demo notebooks to lead with their module — USGS_WaterData_*, USGS_NLDI, USGS_NWIS_WaterUse — and renamed the peak-flow trends demo to peak_streamflow_trends.
• nldi — the nationwide get_features_by_data_source example is marked # doctest: +SKIP (it streams every nwissite feature).
• Sphinx warnings — fixed get_channel bullet-list RST, the duplicated get_monitoring_locations "list of codes" link, the NWIS_Metadata duplicate object description, the deprecated display_version theme option, the missing _static/ directory, and a WaterData_demo heading level.

Verification

• sphinx-build -b doctest: 0 failures (against the live API).
• make html: builds with notebooks executing against the live API — 0 notebook cell errors; the only remaining warnings are the intentional !pip cells in WaterData_demo.
• Notebooks kept output-free (nbsphinx executes them at build time).

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