Update the installation instructions in the documentation

The current installation (and update) instructions are out of date, the
update instructions are not giving any useful information, and the
developer installation instructions are redudundant with the developer
starting guide.

I thus removed most of the instructions and replaced them with the instructions that
are currently in the README.

Author: rlouf

doc/aesara_installer_for_anaconda.bat

@@ -79,7 +79,6 @@ Community
 
    introduction
    install
-   updating
    tutorial/index
    extending/index
    dev_start_guide

doc/index.rst

@@ -3,14 +3,26 @@
 Installing Aesara
 =================
 
-Supported platforms:
+The latest release of Aesara can be installed from Pypi using `pip`:
 
-.. toctree::
-  :maxdepth: 1
+.. code-block:: bash
 
-  install_ubuntu
-  install_macos
-  install_windows
-  install_centos6
+    pip install aesara
 
-To update your current installation see :ref:`updating`.
+
+Or via conda-force:
+
+.. code-block:: bash
+
+    conda install -c conda-forge aesara
+
+
+The current development branch of Aesara can be installed from GitHub using `pip`:
+
+
+.. code-block:: bash
+
+    pip install git+https://github.com/aesara-devs/aesara
+
+
+To use the Numba and JAX backend you will need to install these libraries in addition to Aesara. Please refer to `Numba's installation instructions <https://numba.readthedocs.io/en/stable/user/installing.html>`__ and `JAX's installation instructions  <https://github.com/google/jax#installation>`__ respectively.