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

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+.. _healpix:
+
+********************************************************************************
+HEALPix
+********************************************************************************
++---------------------+----------------------------------------------------------+
+| **Classification**  | Mixed                                                    |
++---------------------+----------------------------------------------------------+
+| **Available forms** | Forward and inverse, spherical and elliptical projection |
++---------------------+----------------------------------------------------------+
+| **Defined area**    | Global                                                   |
++---------------------+----------------------------------------------------------+
+| **Implemented by**  | Alex Raichev and Michael Speth                           |
++---------------------+----------------------------------------------------------+
+| **Options**                                                                    |
++---------------------+----------------------------------------------------------+
+| `No special options for this projection`                                       |
++---------------------+----------------------------------------------------------+
+
+.. image:: ../../../images/healpix.png
+   :scale: 75%
+   :alt:   HEALPix
+
+The HEALPix projection is area preserving and can be used with a spherical and
+ellipsoidal model. It was initially developed for mapping cosmic background
+microwave radiation. The image below is the graphical representation of the
+mapping and consists of eight isomorphic triangular interrupted map graticules.
+The north and south contains four in which straight meridians converge polewards
+to a point and unequally spaced horizontal parallels. HEALPix provides a mapping
+in which points of equal latitude and equally spaced longitude are mapped to points
+of equal latitude and equally spaced longitude with the module of the polar
+interruptions.
+
+
+Usage
+###############################################################################
+
+To run a forward HEALPix projection on a unit sphere model, use the following command::
+
+    proj +proj=healpix +lon_0=0 +a=1 -E <<EOF
+    0 0
+    EOF
+    # output
+    0 0 0.00 0.00
+
+Further reading
+################################################################################
+
+#. `NASA <http://healpix.jpl.nasa.gov/>`_
+#. `Wikipedia <https://en.wikipedia.org/wiki/HEALPix>`_
+
+
+
+
+