docs(README): add mise alternate installation documentation

[jylenhof]

Description 📣

Type ✨

• Bug fix
• New feature
• Improvement
• Breaking change
• Documentation

Tests 🛠️

# Here's some code block to paste some code snippets

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• I have read the contributing guide, agreed and acknowledged the code of conduct. 📝

[greptile-apps]

Greptile Summary

• Adds documentation for installing Infisical CLI using mise-en-place, a polyglot tool version manager
• Provides installation command mise use -g infisical@latest in the README installation section
• Places the mise installation method logically between Arch Linux and Direct Download sections

Important Files Changed

Filename	Overview
README.md	Added documentation for mise-en-place installation method with command example

Confidence score: 5/5

• This PR is safe to merge with no risk as it only adds documentation
• Score reflects the fact that this is a simple documentation addition with no functional changes
• No files require special attention since this is purely documentation

[greptile-apps]

Additional Comments (2)

1. README.md, line 75 (link)

   style: The heading uses 'mise-en-place' but the tool is actually just called 'mise'

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2. README.md, line 77 (link)

   style: Inconsistent backtick usage around 'infisical cli' - other installation methods don't use backticks for the tool name

   _{Note: If this suggestion doesn't match your team's coding style, reply to this and let me know. I'll remember it for next time!}

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