Use a custom layout for language & platform landing pages

[chalin]

This is infrastructure work in support of #433

• [x] Introduce new layout - #482
• [x] Migrate previously-manually-formatted language-home pages - #494
  • [x] Go
  • [x] Java
  • [x] Kotlin
• [ ] Use new layout for remaining language/platform home pages
  • [x] Android
    • [x] Java - #497
    • [x] Kotlin - #496
  • [x] C# (core-library implementation) - #510
  • [x] C++ - #507
  • [x] Dart - #482
  • [ ] Node
  • [ ] Objective-C
  • [x] PHP - #734
  • [x] Python - #508
  • [x] Ruby - #970
  • [ ] Web