Update docs to mention building with --build arg

[maciektr]

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[maciektr]

This change is part of the following stack:

• Add mdbook builder as extension #2140
  • Patch highlighter for Cairo when building with scarb-mdbook #2141
    • Mark code blocks as Cairo not Rust #2142
      • Update docs to mention building with --build arg #2143 ◀

_{Change managed by git-spice.}