Clean up installation page

Author: dstansby

docs/contributing.rst

@@ -1,3 +1,5 @@
+.. _contributing:
+
 Contributing to Zarr
 ====================
 

docs/installation.rst

@@ -1,33 +1,34 @@
 Installation
 ============
 
-Zarr depends on NumPy. It is generally best to `install NumPy
-<https://numpy.org/doc/stable/user/install.html>`_ first using whatever method is most
-appropriate for your operating system and Python distribution. Other dependencies should be
-installed automatically if using one of the installation methods below.
-
-Note: Zarr has endorsed `Scientific-Python SPEC 0 <https://scientific-python.org/specs/spec-0000/>`_ and now follows the version support window as outlined below:
-
-- Python: 36 months after initial release
-- Core package dependencies (e.g. NumPy): 24 months after initial release
-
-Install Zarr from PyPI:
+From PyPI using pip:
 
 .. code-block:: console
 
     $ pip install zarr
 
-Alternatively, install Zarr via conda:
+From conda-forge using conda:
 
 .. code-block:: console
 
     $ conda install -c conda-forge zarr
 
-To install the latest development version of Zarr, you can use pip with the
-latest GitHub main:
+Optional dependencies
+---------------------
+There are a number of optional dependency groups you can install for extra functionality.
+These can be installed using ``pip install "zarr[<extra>]"``, e.g. ``pip install "zarr[gpu]"``
 
-.. code-block:: console
+- ``gpu``: support for GPUs
+- ``fsspec``: support for reading/writing to remote data stores
+- ``tree``: support for pretty printing of directory trees
+
+Dependency support
+------------------
+Zarr has endorsed `Scientific-Python SPEC 0 <https://scientific-python.org/specs/spec-0000/>`_ and now follows the version support window as outlined below:
 
-    $ pip install git+https://github.com/zarr-developers/zarr-python.git
+- Python: 36 months after initial release
+- Core package dependencies (e.g. NumPy): 24 months after initial release
 
-To work with Zarr source code in development, see `Contributing <contributing.html>`_.
+Development
+-----------
+To install the latest development version of Zarr, see :ref:`contributing`.