Use ARG for LLVM_VERSION; update README

[sean-parent]

Replace Dockerfile ENV with ARG for LLVM_VERSION to allow build-time overrides. Update README formatting and Docker usage: add GitHub Packages registry login note, change the docker build example to include memory limits and pass --build-arg LLVM_VERSION=18, convert fenced command blocks to inline commands, and apply minor whitespace/formatting tweaks.

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Motivation and Context

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Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.