Website: update VS Code install instructions

Extensions on the Visual Studio Code marketplace website sometimes don't
have an "Install" button. (The page on my macOS machine does have an
"Install" button, but the page on my Linux Mint machine does not.)
Update our instructions so they make sense if the "Install" button is
missing.

Author: strager

website/public/install/index.ejs.html

@@ -1041,7 +1041,11 @@ <h2>
             href="https://marketplace.visualstudio.com/items?itemName=quick-lint.quick-lint-js"
             >quick-lint-js extension</a
           >
-          page, then click the <em>Install</em> button.
+          page, then click the <em>Install</em> button. If the
+          <em>Install</em> button is missing, copy the
+          <em>Installation</em> command, open Visual Studio Code, press
+          <kbd>CTRL-P</kbd> (Linux or Windows) or <kbd>CMD-P</kbd> (macOS), then
+          paste the copied command.
         </p>
         <p>
           Alternatively, open Visual Studio Code, open the Extensions panel, and