Document dependencies for a for minimal X11 build

We have not documented yet the dependencies for a working X11 build. There should be xorg packages for all linux dependencies, so there should be no need for CDT packages.

xref: https://github.com/conda-forge/matplotlib-feedstock/pull/202

Start by reviewing the documentation structure in this repository and the referenced matplotlib-feedstock PR #202. Identify where build dependencies are documented, then record the Xorg packages needed for a minimal working X11 build and clarify that CDT packages are unnecessary. Done means the dependency guidance is present and unambiguous.

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