Fix logo size and add overview to each chapter

sources/02 Overview.md

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+# Overview
+
+The parts of the Sentinel-2 data that extend the visible spectrum can be used to calculate _Spectral Indices_ or _Spectral Indicators_.
+
+These indicators can be used for a range of different tasks. The Normalised Difference in Water Index (NDWI) for example gives information about how likely a pixel represents water, the Bare-Soil Index (BSI) gives an estimate about how likely a pixel represents unvegetaed soil. Other indices give an indication about vegetation health or plant activity using reflectance properties of the ground in different spectra of light.
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+This chapter explores how to calculate these indices over long and short time spans, leveraging the high spatial resolution to provide highly localized information using these indices.
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+It ends with a notebook that implements the BSI, the NDWI, the Normalized Difference In Vegetation Index (NDVI) and the Normalized Burn Ratio (NBR) which can be used to determine burn severity after a fire. The implementation is extensible enough to perform many different kinds of index calculations on downloaded products with only minimal changes.

sources/02e Spectral Indices.ipynb

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    "outputs": [],
    "source": [
     "indices = {\n",
-    "    # normalized difference in vegetation\n",
-    "    'ndvi': (div, (sub, 'B08', 'B04'), (add, 'B08', 'B04')),\n",
     "    # normalized burn ratio\n",
     "    'nbr': (div, (sub, 'B08', 'B12'), (add, 'B08', 'B12')),\n",
+    "    # normalized difference in vegetation\n",
+    "    'ndvi': (div, (sub, 'B08', 'B04'), (add, 'B08', 'B04')),\n",
+    "    # normalized difference in water index\n",
+    "    'ndwi': (div (sub, 'B03', 'B08'), (add, 'B03', 'B08')),\n",
     "    # bare soil index\n",
     "    'bsi': (div, (sub, (add, 'B11', 'B04'), (add, 'B08', 'B02')),\n",
     "                 (add, (add, 'B11', 'B04'), (add, 'B08', 'B02')))\n",

sources/03 Overview.md

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+# Overview