Create generic user installed software instructions

Add to https://docs.unity.uri.edu/documentation/software/ , or create a new page describing how to use configure --prefix= , cmake -DDCMAKE_INSTALL_PREFIX , and pkg-config when building software in user's directories. Also mention adjusting PATH and LD_LIBRARY_PATH. Inspired by this conversation: https://unity-user-community.slack.com/archives/C04VBHU8QQN/p1713981625253909