GPT install instructions need updating on install page

[Askmarj]

These are very old instructions. https://github.com/MuiseDestiny/zotero-gpt
Zotero desktop (Zotero 7+) doesn't have a tools add-on on the menu. It is now

• Download xpi file on the link above
• Open Zotero on your desktop
• In the top menu bar, click Tools → Plugins (not "Add-ons")
• In the Plugins window, click the gear icon in the top right
• Click "Install Plugin From File" and browse to your downloaded .xpi file

Thank you to Perplexity for the updated instructions :)