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.

[ElliotFriend]

this might need some more work. /home/linuxbrew likely won't exist on a linux system, and (I'm assuming) is a placeholder here. eval "$(~/.linuxbrew/bin/brew shellenv)" might be a better fit

[kaankacar]

Thank you for your feedback Elliot, updated! I looked into it and found that both paths are actually valid:
/home/linuxbrew/.linuxbrew/ is the default for standard installs with sudo, while ~/.linuxbrew/ is used for user-local installs. Went with your suggestion since it's more broadly applicable.

[ElliotFriend]

Interesting! hadn't seen that lol. i even looked at homebrew's linux documentation, but somehow missed that part 🤦🏻‍♂️ now i'm second-guessing myself 🤣

I've never used homebrew on a linux system, so i'm glad to learn something new about it! let's actually revert that change lol. it does appear that the default, best-supported option is to have the installer create and install to /home/linuxbrew, so let's go with that. Thanks!

[ElliotFriend]

Changes introduced in #2103 may simplify any decision we need to make here

[fnando]

In my opinion, suggesting homebrew for linux users is the wrong call. Now that we have the install script, we should refer to that, even if we don't have plans to phase it out.