* Move API-reference links to language-specific pages
* Remove language references from reference page
Note that the page no longer shows up in the left-nav.
* Drop shortcode and data file that aren't used anymore
* Drop reference page
* C# .NET page: make prose match other pages
* gRPC-for-.NET prose tweak
* Eric's feedback: don't add mention of gRPC core in Languages page
* Introduce languages section
* Add Languages to top-nav Docs menu
* Link to Languages page from Docs landing page
* Redirects for quick start and tutorial pages
* Add some leading text for the language list pages
* Tweak Languages list page prose
* Fix language order in Languages page