incorporate collections spec into features spec (#331)

* incorporate collections spec into features spec

* add a few links, add all conformance classes to overview conformance class table

* remove core from conformance class list in each spec, add clarifying text about spec vs. conformance class

* swap Features and Collections to Collections and Features

* remove references to children, which was moved into an extension

CHANGELOG.md

@@ -8,7 +8,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
-- Browseable specification has been incorporated into the *STAC API - Core* specification.
+- The Collections specification has been incorporated into the *Features* specification, but remains as
+  a separate conformance class. 
+- The Browseable specification has been incorporated into the *Core* specification, but remains as
+  a separate conformance class.
 - Extensions moved to standalone specification repositories:
     - [Items and Collections API Version](https://github.com/stac-api-extensions/version) 
     - [Fields](https://github.com/stac-api-extensions/fields)

README.md

@@ -37,15 +37,15 @@ The SpatioTemporal Asset Catalog (STAC) family of specifications aim to standard
 A 'spatiotemporal asset' is any file that represents information about the Earth captured in a certain space and 
 time. The core STAC specifications live in the GitHub repository [radiantearth/stac-spec](https://github.com/radiantearth/stac-spec).
 
-A STAC API is a dynamic version of a SpatioTemporal Asset Catalog. This repository defines the four
-STAC API foundation specifications -- [STAC API - Core](core/), [STAC API - Collections](collections/),
+A STAC API is a dynamic version of a SpatioTemporal Asset Catalog. This repository defines the three
+STAC API foundation specifications -- [STAC API - Core](core/),
 [STAC API - Features](ogcapi-features/), and [STAC API - Item Search](item-search/) -- which can be composed 
 with [Extensions](extensions.md) to define a specific STAC API implementation.
 
 A STAC API can be used to retrieve STAC [Catalog](stac-spec/catalog-spec/catalog-spec.md), 
 [Collection](stac-spec/collection-spec/collection-spec.md), [Item](stac-spec/item-spec/item-spec.md), 
 or STAC API [ItemCollection](fragments/itemcollection/README.md) objects from various endpoints.
-Catalog and Collection objects are JSON, while Item and ItemCollection objects are GeoJSON-compliant entities with foreign members.  
+Catalog and Collection objects are JSON, while Item and ItemCollection objects are GeoJSON-compliant entities with foreign members.
 Typically, a Feature is used when returning a single Item object, and FeatureCollection when multiple Item objects (rather than a 
 JSON array of Item entities).
 
@@ -104,9 +104,6 @@ The **[Overview](overview.md)** document describes all the various parts of the
 The *[core](core/)* folder describes the core STAC API specification that enables browsing catalogs and 
 retrieving the API capabilities. This includes the OpenAPI schemas for STAC Item, Catalog and Collection objects.
 
-**STAC API - Collections:**
-The *[collections](collections)* folder describes how a STAC API Catalog can advertise the Collections it contains.
-
 **STAC API - Features:**
 The *[ogcapi-features](ogcapi-features)* folder describes how a STAC API can fully implement [OGC API - 
 Features](http://docs.opengeospatial.org/is/17-069r3/17-069r3.html) to expose individual `items` endpoints for search of