Update the mobile_guide page to the new design and link out to Element X by default. (#30172)

* Reapply "Update the mobile_guide page to the new design. (#30006)" (#30104)

This reverts commit c51823db5e.

* Use Element X as the default mobile_guide_app_variant when omitted.

* Fix a build error on Windows.

Additionally revert "Remove unnecessary <%= require %> usages" and let webpack handle all of the assets (without a manual copy rule).

* Exclude mobile_guide from coverage gate

It has playwright tests

* Revert the re-introduction of <%= require %>

* Fix snapshot tests on mobile_guide.

---------

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

webpack.config.js

@@ -679,6 +679,12 @@ module.exports = (env, argv) => {
                         context: path.resolve(__dirname, "node_modules/@element-hq/element-call-embedded/dist"),
                         to: path.join(__dirname, "webapp", "widgets", "element-call"),
                     },
+                    // Mobile guide assets
+                    {
+                        from: "assets/**",
+                        context: path.resolve(__dirname, "src/vector/mobile_guide"),
+                        to: "mobile_guide",
+                    },
                 ],
             }),
 
@@ -771,6 +777,8 @@ module.exports = (env, argv) => {
 function getAssetOutputPath(url, resourcePath) {
     const isKaTeX = resourcePath.includes("KaTeX");
     const isFontSource = resourcePath.includes("@fontsource");
+    const mobileGuideAssetsPath = path.join("mobile_guide", "assets");
+    const isMobileGuide = resourcePath.includes(mobileGuideAssetsPath);
     // `res` is the parent dir for our own assets in various layers
     // `dist` is the parent dir for KaTeX assets
     // `files` is the parent dir for @fontsource assets
@@ -804,6 +812,11 @@ function getAssetOutputPath(url, resourcePath) {
         outputDir = "fonts";
     }
 
+    if (isMobileGuide) {
+        // Specific handling for the mobile guide assets, as they live alongside the page sources.
+        outputDir = mobileGuideAssetsPath;
+    }
+
     if (isKaTeX) {
         // Add a clearly named directory segment, rather than leaving the KaTeX
         // assets loose in each asset type directory.