Missing json and toml in config on getting started on v2 that was in v1

[kaosine]

Not sure where to have put this, but I noticed that on the docs here:

https://v2.vuepress.vuejs.org/guide/page.html#frontmatter

That it's missing this from v1:

https://vuepress.vuejs.org/guide/frontmatter.html#alternative-frontmatter-formats

Even though, that JSON part (since that's what I went with) still work as far as I can tell. This needs to be migrated to the newer docs for clarity’s sake. Plus JSON and toml are just more readable with what you can do with them. But I think they need to be better documented and using mrhope's tabs if possible to clean up things too.