[SUBS] Dedicated Documentation for the Checkout Widget

[kasparkallas]

• [x] Set up static site generation with ~Docusaurus~ Nextra
• [x] Set up site deployment with Vercel
• [x] Set up Tailwind CSS for better styling options?
• [x] Set up support channel links in the header (Discord's #dev-support and GitHub)
• [x] Set up link to NPM package in the header with the package version
• [ ] Set up changelog?
• [x] Import the pages from GitBook
• [ ] Set up reference docs inclusion using TypeDoc plugin: https://www.npmjs.com/package/docusaurus-plugin-typedoc/v/0.17.5
• [x] Document eNFT?
• [ ] Set up Showcase (e.g. MadFi)?