Instructions for cross-platfrom builds are incorrect

# 🐛 Summary #

In `README.md`, there are [instructions for performing a cross-platform build locally](https://github.com/cisagov/skeleton-docker/#cross-platform-builds). These instructions are currently incorrect, since the `buildx-dockerfile.sh` script they reference does not exist.

In addition, one would expect that doing such a thing would require installing some tooling locally but this is not mentioned.

These instructions should be corrected or removed.

Start in the Cross-platform builds section of README.md and verify whether buildx-dockerfile.sh exists and what local tooling the instructions require. Done means the instructions are corrected with accurate tooling details, or the obsolete section is removed.

Written by the indexing model from the issue text.