Add idaes-extensions step to getting started for devs (#1129)

* revise readthedocs getting started

* empty commit to check editor

* finish revisions

docs/getting_started.rst

@@ -155,8 +155,14 @@ Install WaterTAP and the development dependencies using ``pip`` and the ``requir
 
    pip install -r requirements-dev.txt
 
-If needed, follow the steps described above to install solvers distributed through IDAES Extensions.
+If needed, or if this is your first time installing IDAES or WaterTAP on your machine, run the following line from the same environment where WaterTAP was installed.
 
+.. code-block:: shell
+
+   idaes get-extensions
+
+.. note:: Typically, the ``idaes get-extensions`` command only needs to be run once for each system, as it will install the required files into a common, system-wide location.  Depending on your operating system, you may need to follow additional steps described above to install solvers distributed through IDAES Extensions.
+
 (Optional but recommended) `Pre-commit hooks <https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks>`_ are scripts that are automatically run by Git "client-side" (i.e. on a developer's local machine) whenever `git commit` is run. WaterTAP uses the `pre-commit <https://pre-commit.com/>`_ framework to manage a few hooks that are useful for WaterTAP developers. To install the WaterTAP pre-commit hooks, run:
 
 .. code-block:: shell