Editora 0.20.0. Archived, so links from that release keep working. Read the 0.24.0 page →
Markdown: Editor and Preview
view.markdownSplit Show the Markdown editor and preview side by side.
How to run it
Open the command palette (M-x in the Emacs keymap) and search for Markdown: Editor and Preview, then press Enter. It has no default keybinding, but you can bind one in Settings → Keymaps.