Tables and a synced split view in docs

Two upgrades for writing docs:

• Tables. Pick a size from the grid in the toolbar and a ready-to-fill table drops into the page, styled cleanly in both themes.
• A synced split view. In split view, moving your cursor in the Markdown source highlights the matching spot in the preview, and double-clicking a word in the preview jumps your cursor straight to it in the source.
• Linkable headings. Every heading now has its own anchor, so you can link to a specific section of a page.