Feature suggestions: Keyboard shortcuts ad UX improvement

There are some suggestions of mine to improvement Markdown editing:

1. Hit `Ctrl+K` to insert hyperlink to the current selection.

2. When hit **Hyperlink** tool button, the cursor should stop at the link and the default `https://` should be removed for been selected. So that I can just paste link to the doc and complete the hyperlink insertion.

3. Hit `Ctrl+shift+/` can show Help window and display supported keyboard shortcuts on the screen. It's very common in many web platform such as Dropbox Paper or Google Docs,