Merge #15964: Docs: Improve build-osx document formatting

dbd137a4ea8f1b5dfc5cdc72cee99c8f8328b793 Improve build-osx formatting (Giulio Lombardo)

Pull request description:

  This `PR` will improve `build-osx.md` formatting by:

  1. Updating Markdown syntax to the latest one
  2. Adding syntax highlighting to all code blocks
  3. Aligning the text up to `80` column guideline (before it was following different guidelines, sometime `80`, sometime `90`, etc.)
  4. Small grammar improvements here and there

ACKs for top commit:
  fanquake:
    ACK dbd137a4ea8f1b5dfc5cdc72cee99c8f8328b793 - Document reads and renders essentially the same as the current `build-osx.md`, with minor formatting / grammatical  changes.

Tree-SHA512: 47747991b5fddf0725c82f17f153e83150e51f698787544b4c51b32479989e4b550e2b3aec92979d2b0c76edfdcbbe7c4d9d0115df12e2bfde0cfcb277e9b984

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