Move 'Coordinate operations' to top level of docs [skip ci]

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+.. _vgridshift:
+
+================================================================================
+Vertical grid shift
+================================================================================
+
+Change Vertical datum change by grid shift
+
++-----------------+--------------------------------------------------------------------+
+| **Input type**  | Geodetic coordinates (horizontal), meters (vertical).              |
++-----------------+--------------------------------------------------------------------+
+| **Output type** | Geodetic coordinates (horizontal), meters (vertical).              |
++-----------------+--------------------------------------------------------------------+
+| **Options**                                                                          |
++-----------------+--------------------------------------------------------------------+
+| `+grids`        | Comma-separated list of grids to load.                             |
++-----------------+--------------------------------------------------------------------+
+
+The vertical grid shift is done by offsetting the vertical input coordinates by
+a specific amount determined by the loaded grids. The simplest use case of the
+horizontal grid shift is applying a single grid. Here we change the vertical
+reference from the ellipsoid to the global geoid model, EGM96::
+
+    +vgridshift +grids=egm96_16.gtx
+
+
+More than one grid can be loaded at the same time, for instance in the case where
+a better geoid model than the global is available for a certain area. Here the
+gridshift is set up so that the local DVR90 geoid model takes presedence over
+the global model::
+
+    +vgridshift +grids=@dvr90.gtx,egm96_16.gtx
+
+The ``@`` in the above example states that the grid is optional, in case the grid
+is not found in the PROJ search path. The list of grids is prioritized so that
+grids in the start of the list takes presedence over the grids in the back of the
+list.
+
+PROJ supports the GTX file format for vertical grid corrections. Details
+about all the format can be found in the GDAL documentation. GDAL both reads and
+writes the format. Using GDAL for construction of new grids is recommended.