Editing existing pages is very easy, just go to it on the website then press the "Help us improve this page" link in the bottom left of it. I recommend previewing the code in using the markdown-it live editor as they're functionally similiar.
Pretty much just keep the current structure, preferably don't add anything under FAQ, just edit the README.md for that. If you're adding new guides then go to nav.js and edit it in, follow the current example of Getting started, shouldn't be too hard.
Pretty much just fork the repo, clone it to your computer then do a npm install then vuepress dev src/npm run serve.
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