Fixup issue with .ipynb extension

Sphinx/myst_nb seems to have problems with including the extension. I get xref
missing warnings when they are present. It also causes relative links with
`.ipynb` to not render at all in the docs.

This represents a trade-off: the docs will have nicely rendered links to the
other notebooks. For users who download the files themselves and execute them as
notebooks locally, the links will not work. However, it should be clear enough
what they refer to.

Author: trey-stafford

docs/notebooks/index.md

@@ -5,10 +5,10 @@
 :hidden:
 :caption: content
 
-./0_introduction.ipynb
-./corrections.ipynb
-./iceflow-example.ipynb
-./iceflow-with-icepyx.ipynb
+./0_introduction
+./corrections
+./iceflow-example
+./iceflow-with-icepyx
 ```
 
 [Jupyter notebooks](https://docs.jupyter.org/en/latest/) provide executable

notebooks/0_introduction.ipynb

@@ -36,7 +36,7 @@
     "* Data from the Airborne Topographic Mapper (ATM) flown during the IceBridge campaigns is stored in four different file formats. ICESat/GLAS and ICESat-2 data are also in different file formats. Therefore, the data needs to be harmonized, that means placed into similar formats before comparisons can be made.\n",
     "* The coordinate reference systems used to locate measurements have changed over the years, as the Earth's surface is not static and changes shape. To account for these changes, terrestrial reference frames that relate latitude and longitude to points on the Earth are updated on a regular basis. Since the launch of ICESat/GLAS, the International Terrestrial Reference Frame [(ITRF)](https://www.iers.org/IERS/EN/DataProducts/ITRF/itrf.html) has been updated three times. The geolocation of a point measured at the beginning of the record and the end of the record is not the same even though the latitude and longitude is the same. These changes in geolocation need to be reconciled to allow meaningful comparisons within the long-term data record.\n",
     "\n",
-    "The companion [corrections.ipynb](./corrections.ipynb) notebook contains information about how to perform ITRF transformations using [PyProj](https://pyproj4.github.io/pyproj/stable/index.html).\n",
+    "The companion [Applying Coordinate Transformations to Facilitate Data Comparison](./corrections) notebook contains information about how to perform ITRF transformations using [PyProj](https://pyproj4.github.io/pyproj/stable/index.html).\n",
     "\n",
     "## **2.1 Pre-IceBridge**\n",
     "\n",
@@ -107,7 +107,7 @@
     "\n",
     "# 6. Next steps\n",
     "\n",
-    "With the knowledge gained in this notebook, users should be prepared for the [iceflow-example.ipynb](./iceflow-example.ipynb) notebook, which provides an example of how to search for, download, and interact with `ILATM1B v1` data for a small area of interest with the `iceflow` library."
+    "With the knowledge gained in this notebook, users should be prepared for the [NSIDC Iceflow example](./iceflow-example) notebook, which provides an example of how to search for, download, and interact with `ILATM1B v1` data for a small area of interest with the `iceflow` library."
    ]
   }
  ],

notebooks/iceflow-example.ipynb

@@ -7,13 +7,13 @@
    "source": [
     "# NSIDC Iceflow example\n",
     "\n",
-    "This notebook shows an example of how to use the `nsidc-iceflow` Python library to do ITRF transformations with real data. We recommend starting with the [corrections.ipynb](./corrections.ipynb) notebook to learn more about ITRF transformations and why they matter. \n",
+    "This notebook shows an example of how to use the `nsidc-iceflow` Python library to do ITRF transformations with real data. We recommend starting with the [Applying Coordinate Transformations to Facilitate Data Comparison](./corrections) notebook to learn more about ITRF transformations and why they matter. \n",
     "\n",
     "Lets assume we want to do an analysis using [IceBridge ATM L1B Qfit Elevation and Return Strength, Version 1 (ILATM1B)](https://nsidc.org/data/ilatm1b/versions/1) data near Pine Island Glacier in Antarctica.\n",
     "\n",
     "Finding, downloading, and reading ILATM1B v1 data with `nsidc-iceflow` is straightforward. ILATM1B data can be very large, so for the purposes of this example we will focus on just a small area near Pine Island Glacier along with a short date range in order to fetch a manageable amount of data.\n",
     "\n",
-    "To learn about how to download and manage larger amounts of data across many datasets with `nsidc-iceflow`, see the [Using iceflow with icepyx to Generate an Elevation Timeseries](./iceflow-with-icepyx.ipynb) notebook."
+    "To learn about how to download and manage larger amounts of data across many datasets with `nsidc-iceflow`, see the [Using iceflow with icepyx to Generate an Elevation Timeseries](./iceflow-with-icepyx) notebook."
    ]
   },
   {