Duplicate install instructions

[abachma2]

Looking at source/user/install.rst, there are multiple links for each type of install. There is a list of the install types, with some recommendations, and there is a toctree with the file names. So this creates a duplicate of the list. Some paths on how to remedy this:

• Leave the :doc: elements with the recommendations in install.rst, and remove the doc tree
• Remove the :doc: elements, leaving the toctree in install.rst. This would likely involve moving the recommendations into each of the files to keep that information present for users.

[bennibbelink]

I personally think that it is helpful to have the recommendations, so I'm partial to the first bullet above. It would also require that we add a :doc: element for the Docker Install option.

Could we possibly remove the references to "Custom" and "CMake" Cyclus Installations from install.rst since they are only relevant to installing from source? Both the "via Tarball" and "via Repository" pages reference the custom build options already.

[abachma2]

That's another thing I modify for this.

[abachma2]

I believe this was addressed in #362, so I'm closing it.