Add Arch Linux User Repository (AUR) installation instructions to documentation

[LukeLabrie]

Description

This PR adds an installation option to the User's Guide for Arch Linux and other Arch-based distro users through the Arch Linux User Repository (AUR).

The goal here is to provide a one-step, Arch-flavored installation option that delivers a fully featured stack including:

• DAGMC and MPI enabled version of OpenMC which tracks the develop branch
• User-selected nuclear data installed to /opt
• CAD_to_OpenMC meshing tool

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @LukeLabrie!