docs: Added warning about Vercel branch deployment

[ArturCharylo]

Description

Added a note to the Vercel installation section to remind users that they must deploy the 'vercel' branch to avoid 404 errors.

Type of change

• Updated documentation (updated the readme, templates, or other repo files)

Checklist:

• I have checked to make sure no other pull requests are open for this issue
• The code is properly formatted and is consistent with the existing code style
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[DenverCoder1]

Thank you!