Add Web API controller definition doc (#6003)
* Add Web API controller definition doc
* Remove wwwroot folder reference from csproj file
* Verbiage tweaks
* Add info about FromForm inference
* Bold the attribute names
* Add TOC entries
* Change link ordering in TOC
* Add note about IntelliSense
* Major sample app changes
* Fix code snippet line numbers
* Minor verbiage tweaks
* Add a README file
* Add TestController.cs
* Verbiage changes
* Shorten wording
* Change wording
* Minor edit
* React to feedback
* Add attributes table
* Verbiage tweaks
* querystring -> query string
* Minor edit
* Add 2.1 controller samples
* Fix invalid moniker names
* Update code sample to 2.1
* Update line number
* Convert uppercase W to lowercase
* React to feedback
* Update sample app's README file
* Project file cleanup
* Replace word
* Remove MVC views from sample app
* Derive from ControllerBase instead of Controller
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