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>

sonar-project.properties

@@ -19,5 +19,6 @@ sonar.coverage.exclusions=\
   src/vector/modernizr.js,\
   src/components/views/dialogs/devtools/**/*,\
   src/utils/SessionLock.ts,\
-  src/**/*.d.ts
+  src/**/*.d.ts,\
+  src/vector/mobile_guide/**/*
 sonar.testExecutionReportPaths=coverage/jest-sonar-report.xml