Update reconfig instructions to explain how to use board files with Alveo U50 and U280

[jyoung3131]

We need a page that talks about using board files with the Vivado flow likely going on this page: https://github.com/gt-crnch-rg/read-the-docs/blob/main/docs/reconfig/xilinx/xilinx-vivado-flow.rst

[jyoung3131]

Initial updates to this page are now complete. We may want to update some more as we discuss so I'm going to leave this issue open.