Add Taiwan fetcher

[thiagovmdon]

Summary

Related to issue #114
Adds a Taiwan fetcher based on the Water Resources Agency (WRA).

The implementation uses:

• the official WRA OpenAPI for station metadata
• the public WRA HydroInfo endpoints for historical discharge and water-level time series

Supported variables:

• DISCHARGE_DAILY_MEAN
• DISCHARGE_HOURLY_MEAN
• STAGE_DAILY_MEAN
• STAGE_HOURLY_MEAN

Data source

• Metadata: https://opendata.wra.gov.tw/openapi/swagger/index.html
• Historical values: https://gweb.wra.gov.tw/HydroInfo/
• Terms of use: https://data.gov.tw/en/license

Implementation notes

• Metadata is downloaded from the official WRA OpenAPI and standardized to RivRetrieve metadata fields.
• TWD97 coordinates are converted to latitude/longitude.
• Historical discharge and water-level values are retrieved from HydroInfo JSON endpoints.
• HydroInfo water-level values appear to be absolute elevations, so stage is converted using the station zero-point elevation from metadata.
• Original provider metadata fields are preserved in addition to the standardized columns.

Docs

• Added Taiwan API docs page.
• Kept the rendered Sphinx page aligned with the Germany Berlin structure:
  • Data source
  • Supported variables
  • Data description and API
  • Terms of use

Caveats

• The metadata endpoint is part of the official WRA OpenAPI.
• The historical HydroInfo endpoints are public and used by the website, but do not appear to be documented in the official Swagger spec.
• Historical coverage is station-specific and may be sparse for some gauges.