docs: fix WQP README example unpacking and stale install instructions

README WQP examples assigned the (DataFrame, metadata) tuple to a single name,
so `len(sites)` / `len(results)` printed 2 (the tuple length) instead of the
row count. Unpack to `sites, metadata = ...` / `results, metadata = ...`
(verified live: prints 25573 sites / 163 measurements).

docs/source/meta/installing.rst referenced requirements.txt and
requirements-dev.txt, which no longer exist — dependencies moved to
pyproject.toml. Point the intro at [project.dependencies] and
[project.optional-dependencies], replace the dev-install step with
`pip install -e ".[test,doc,nldi]"`, and drop the two dangling rst link
targets.

The NLDI README examples (feature_source='comid') were checked against the
live NLDI API and work as written (get_basin -> 1 feature, get_flowlines -> 7
flowlines), so they are intentionally left unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: thodson-usgs

README.md

@@ -125,16 +125,16 @@ Access water quality data from multiple agencies:
 ```python
 from dataretrieval import wqp
 
-# Find water quality monitoring sites
-sites = wqp.what_sites(
+# Find water quality monitoring sites (returns a DataFrame and metadata)
+sites, metadata = wqp.what_sites(
     statecode='US:55',  # Wisconsin
     siteType='Stream'
 )
 
 print(f"Found {len(sites)} stream monitoring sites in Wisconsin")
 
 # Get water quality results
-results = wqp.get_results(
+results, metadata = wqp.get_results(
     siteid='USGS-05427718',
     characteristicName='Temperature, water'
 )

docs/source/meta/installing.rst

@@ -3,13 +3,9 @@ Installation Guide
 
 Whether you are a user or developer we recommend installing ``dataretrieval``
 in a virtual environment. This can be done using something like ``virtualenv``
-or ``conda``. Package dependencies are listed in the `requirements.txt`_ file,
-a full list of dependencies necessary for development are listed in the
-`requirements-dev.txt`_ file.
-
-.. _requirements.txt: https://github.com/DOI-USGS/dataretrieval-python/blob/main/requirements.txt
-
-.. _requirements-dev.txt: https://github.com/DOI-USGS/dataretrieval-python/blob/main/requirements-dev.txt
+or ``conda``. Package dependencies are declared in ``pyproject.toml``: the core
+runtime dependencies under ``[project.dependencies]``, and optional extras
+(``test``, ``doc``, ``nldi``) under ``[project.optional-dependencies]``.
 
 
 User Installation
@@ -59,8 +55,11 @@ the package for development:
 
 .. code-block:: bash
 
-    $ pip install -r requirements-dev.txt
-    $ pip install -e .
+    $ pip install -e ".[test,doc,nldi]"
+
+This installs ``dataretrieval`` in editable mode along with the development
+extras: ``test`` (test runner), ``doc`` (documentation build), and ``nldi``
+(``geopandas``, required by the NLDI module).
 
 To check your installation you can run the tests with the following commands: