Closes#314. As mentioned in #314, this will help us get better analytics for API-reference visits.
* Update API links
* Update links to the tutorial
* Update links to intro and concepts
* Add "What is gRPC?" section
* Move "core concepts" page to new section
* Move "What is gRPC" text to Intro page
* List techniques in Guides landing page
* Add "What is gRPC?" to Docs drop-down
* Link "Learn more" CTA to intro page
* Replace "What is gRPC?" links with "Intro" links
Replace links to
[What is gRPC?](guides)
by
[Introduction to gRPC](what-is-grpc/introduction)
* Replace links to "Overview" by links to Intro
Formerly the "Overview" page contained the text that is now in the intro page.
* Add redirect for concepts page
* Fix link in intro page
* 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
Patrice Chalin
Luc Perkins
Patrice Chalin
Richard Belleville