https://github.com/leanprover/vscode-lean4/pull/521 changed the display
name of the VS Code extension so that it can be found more easily when
searching for "Lean" (before it would appear far down in the list). This
PR updates the quickstart guide to reflect this fact.
This PR updates the screenshots and instructions in the quickstart guide
for the most recent Lean 4 VS Code extension version and makes a small
stylistic change suggested by @semorrison.