Improve overview.rst with narrative and Quick Start example

changelog/165.doc.rst

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+Overview Documentation Update
+=============================
+
+- Improved the Overview section of the documentation (overview.rst)
+- Added a clear description of radiospectra
+- Listed all supported instruments with official names
+- Added a verified Quick Start example for e-CALLISTO using the public Spectrogram API
+- Explained the workflow in plain English for new users

docs/overview.rst

@@ -1,12 +1,60 @@
-********
 Overview
-********
+========
 
-``radiospectra`` currently supports reading spectra from:
+radiospectra is a Python package for reading and working with
+solar and heliospheric radio spectrogram data. It provides a
+common interface for loading dynamic spectra from multiple
+radio instruments and representing them as spectrogram objects.
 
-- e-CALISTO
-- Extend Owen Valley Array
-- Parker Solar Probe FIELDS/Radio Frequency Spectrometer
-- Radio Solar Telescope Network
-- STEREO Waves
-- Wind Waves
+Spectrogram data are represented using a unified object model,
+allowing users to inspect metadata, access intensity data,
+and generate visualizations in a consistent way across
+different instruments.
+
+Supported Instruments
+---------------------
+
+radiospectra currently supports spectrogram data from:
+
+- e-CALLISTO
+- Expanded Owens Valley Solar Array (EOVSA)
+- Parker Solar Probe FIELDS/RFS
+- Radio Solar Telescope Network (RSTN)
+- STEREO/WAVES
+- Wind/WAVES
+
+Quick Start
+-----------
+
+The example below demonstrates how to search, download,
+and plot e-CALLISTO spectrogram data.
+
+.. code-block:: python
+
+    import radiospectra.net
+    from sunpy.net import Fido, attrs as a
+    from radiospectra.spectrogram import Spectrogram
+    from radiospectra.net import attrs as ra
+
+    # Search for e-CALLISTO data
+    query = Fido.search(
+        a.Time('2019/10/05 23:00', '2019/10/06 00:59'),
+        a.Instrument('eCALLISTO'),
+        ra.Observatory('ALASKA')
+    )
+
+    # Download the first result
+    downloaded = Fido.fetch(query[0][0])
+
+    # Create a spectrogram object
+    spec = Spectrogram(downloaded[0])
+
+    # Plot the spectrogram
+    spec.plot()
+
+This workflow:
+
+- Searches the e-CALLISTO archive
+- Downloads a spectrogram file
+- Loads it into a Spectrogram object
+- Displays a frequency–time intensity plot