feat(v2): add 'custom_edit_url' and 'hide_title' markdown header feature (#1838)

* feat(v2): add 'custom_edit_url' and 'hide_title' markdown header feature

* nits

website/docs/markdown-features.mdx

@@ -92,6 +92,28 @@ The headers are well-spaced so that the hierarchy is clear.
 
 </BrowserWindow>
 
+### Markdown Headers
+
+Documents use the following markdown header fields that are enclosed by a line `---` on either side:
+
+- `id`: A unique document id. If this field is not present, the document's `id` will default to its file name (without the extension).
+- `title`: The title of your document. If this field is not present, the document's `title` will default to its `id`.
+- `hide_title`: Whether to hide the title at the top of the doc. By default it is `false`.
+- `sidebar_label`: The text shown in the document sidebar and in the next/previous button for this document. If this field is not present, the document's `sidebar_label` will default to its `title`.
+- `custom_edit_url`: The URL for editing this document. If this field is not present, the document's edit URL will fall back to `editUrl` from options fields passed to `docusaurus-plugin-content-docs`.
+
+Example:
+
+```md
+---
+id: doc-markdown
+title: Markdown Features
+sidebar_label: Markdown :)
+custom_edit_url: https://github.com/facebook/docusaurus/edit/master/docs/api-doc-markdown.md
+---
+
+```
+
 ### Embedding React components
 
 Docusaurus has built-in support for [MDX](https://mdxjs.com), which allows you to write JSX within your Markdown files and render them as React components.