docs: Add Homebrew installation instructions for Linux

[kaankacar]

Problem

The current documentation assumes brew is available on Linux or that the user knows how to install it. An audit of the "Getting Started" guide identified this as a blocker for users on Linux, where commands like brew install stellar-cli fail with "command not found".

Solution

This PR adds explicit instructions for installing Homebrew on Linux within the setup.mdx file.

Changes:

• Added the standard Homebrew installation command for Linux.
• Added the command to configure the shell environment.

Verification

• Verified that the commands are the standard installation steps from brew.sh.
• The changes are restricted to the Linux tab in the Install the Stellar CLI section.