Unit conversion docs

[javerbukh]

Documentation for unit conversion.

[javerbukh]

@brechmos-stsci @eteq I have written some documentation for unit conversion, however this is my first time writing a .rst file, so I'm not sure if this is how it should look. Can you guys let me know about any changes that need to be made?

[nmearl]

Just a few minor things to keep in mind.

[eteq]

High-level comment: if you use the >>> syntax instead of just relying on .. code-block::, code in .rst files get run as doctests. That is, when you run the tests, it will run any code in the >>> section and make sure it responds as expected. That makes sure the examples are actually accurate. By >>> syntax I mean:

.. code-block:: python

    >>> import astropy.units as u
    >>> potential_X_axis_units = ...

That said, I'm not entirely sure that's been turned on in specviz yet, but I find it's usually a good idea to do that so that things are ready if/when it is.

[nluetzge]

How about adding a screenshot to to the main description? Just something like that:

Otherwise it looks good to me.