Adding homebrew installation to documentation

[nathanfallet]

I added a simple paragraph in the SDK page to show how to install Flutter using Homebrew, which is a preferred method for many developers using macOS.

See https://formulae.brew.sh/cask/flutter for details about the Homebrew cask.

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[nathanfallet]

@googlebot I signed it!

[sfshaza2]

LGTM, with a couple of minor edits.