docs: Add --parallel flag to cmake build commands in BUILD.md

[ricky122-5]

High Level Overview of Change

Added the --parallel N flag to the cmake --build commands in BUILD.md to document how to speed up builds using parallel jobs.

Context of Change

The build documentation didn't mention the --parallel flag, which is a common way to significantly reduce build times. This is a documentation-only change with no code impact.

API Impact

No impact. Only a documentation change.

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

[github-actions]

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[mvadari]

@ricky122-5 please fix the pre-commit issues and then let us know if it's ready to be merged