Clean up README and drop unused requirements-dev.txt

[thodson-usgs]

Summary

• Drop requirements-dev.txt — the file isn't referenced by any CI workflow or script. Dev dependencies are already declared in pyproject.toml under the [test], [doc], and [nldi] extras and installed in CI via pip install .[test,nldi] / .[doc,nldi].
• Streamline the README:
  • Move the announcements block to NEWS.md (link kept from the intro).
  • Drop the redundant legacy NWIS usage example; the NWIS module is deprecated and the modern waterdata examples already cover the same ground.
  • Move the API-key callout next to the Water Data section where it's actually relevant.
  • Tighten wording in the example intros and the logging blurb (consistent imperative voice, drop the puzzling "and collections" phrase, lower-case fix on "python").
  • Fix two broken GitHub links that still pointed at the retired USGS-python/dataretrieval org → DOI-USGS/dataretrieval-python.
  • Replace the "Documentation: available in source code docstrings" entry with a direct link to the existing Sphinx site at https://doi-usgs.github.io/dataretrieval-python/.

No functional or packaging changes — README and a stale dev-requirements file only.

Test plan

• Render the README on GitHub and confirm the code blocks, headings, and links display correctly
• Click each updated link (demos directory, issue tracker, documentation site) to confirm it resolves
• Confirm CI still passes (workflows install via pip install .[test,nldi] / .[doc,nldi], not via the deleted file)

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