docs: add @reown/appkit installation step

[helciofranco]

• Relates to FE-1397

Summary

📷 Demo

Checklist

• I've added error handling for all actions/requests, and verified how this error will show on UI. (or there was no error handling)
• I've reviewed all the copy changed/added in this PR, using AI if needed. (or there was no copy changes)
• I've included the reference to the issues being closed from Github and/or Linear (or there was no issues)
• I've changed the Docs to reflect my changes (or it was not needed)
• I've put docs links where it may be helpful (or it was not needed)
• I checked the resulting UI both in Light and Dark mode (or no UI changes were made)
• I reviewed the entire PR myself (preferably, on GH UI)