Simplify VS Code setup caption and fix extension troubleshooting

[GuanzhouSong]

"Set up in VS Code" fails silently or with a confusing error for several groups of users, and the caption promised a prompt that doesn't always come:

• Extension not installed, managed device (private marketplace policy): VS Code's install-from-link fails with No extension gallery service configured and never starts setup. It fails with VS Code already running too, so the guide's "open the link again once VS Code has loaded" advice was wrong.
• Extension older than 0.10.1: old versions route on the query only, so /local fails with "Failed to process URI: A DocumentDB deep-link was opened without a connection string".
• Updated but not restarted: the running window keeps the old version's handler, so the same error shows until VS Code restarts.
• code --install-extension over an old version prints "already installed" and doesn't update without --force.

Reproduced each with VS Code 1.139 and extension 0.10.0/0.10.2, using VS Code's logs and an isolated user-data dir.

The website can't detect the extension or its version, and a link can't chain install-then-setup, so the homepage keeps one button. The caption now only covers what the button can't: "Don't have VS Code? Download it first." Recovery for the cases above goes in the guide's install and troubleshooting sections, per the earlier review ask to keep troubleshooting off the homepage.