docs: Add Homebrew installation instructions for Linux

[google-labs-jules]

This PR updates the setup guide to include instructions for installing Homebrew on Linux. This addresses an issue where users on Linux without Homebrew installed were blocked when trying to install the Stellar CLI using brew.

Changes:

• Added Homebrew installation command to the Linux tab in setup.mdx.
• Added command to configure the shell environment for Homebrew.

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PR created automatically by Jules for task 13623249929182014483 started by @kaankacar

[google-labs-jules]

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