Add a showMarkdownEditButton flag to themeConfig

[rakannimer]

In the header https://github.com/doczjs/docz/blob/master/core/gatsby-theme-docz/src/components/Header/index.js#L15 the code assumes that Entry has an edit boolean field and defaults it to true.

However, entry doesn't have an edit flag.

2 suggestions :

1. Add themeConfig.showMarkdownEditButton to docz config and default it to true.
2. Add edit to Entry and read it from the header of the current mdx file.