add extensions section to ocgapi-features, cleanup other extensions sections (#181)

Author: philvarner

core/README.md

@@ -3,6 +3,7 @@
 - [STAC API - Core Specification](#stac-api---core-specification)
   - [Link Relations](#link-relations)
   - [Example Landing Page for STAC API - Core](#example-landing-page-for-stac-api---core)
+  - [Extensions](#extensions)
 
 - **OpenAPI specification:** [openapi.yaml](openapi.yaml) describes the core endpoints ([rendered version](https://api.stacspec.org/v1.0.0-beta.2/core)),
   and [commons.yaml](commons.yaml) is the OpenAPI version of the core [STAC spec](../stac-spec) JSON Schemas.
@@ -117,3 +118,7 @@ the [overview](../overview.md#example-landing-page) document.
     ]
 }
 ```
+
+## Extensions
+
+None.

item-search/README.md

@@ -78,9 +78,10 @@ Implementations that support `POST` should add a second link with the same struc
 
 ## Query Parameters and Fields
 
-The following list of parameters is used to narrow search queries. They can all be represented as query string parameters 
-in a GET request, or as JSON entity fields in a POST request. For filters that represent a set of values, query parameters 
-should use comma-separated string values with no enclosing brackets (\[ or \]) and no whitespace between values, and JSON entity attributes should use JSON Arrays. 
+The following list of parameters is used to narrow search queries. They can all be represented as query 
+string parameters in a GET request, or as JSON entity fields in a POST request. For filters that represent 
+a set of values, query parameters should use comma-separated string values with no enclosing brackets 
+(\[ or \]) and no whitespace between values, and JSON entity attributes should use JSON Arrays. 
 
 ### Query Examples
 
@@ -286,7 +287,7 @@ the [overview](../overview.md#example-landing-page) document.
 
 ## Extensions
 
-These extensions provide additional functionality that enhances the core item search. All are specified as 
+These extensions provide additional functionality that enhances Item Search. All are specified as 
 [fragments](../fragments), as they are re-used by other extensions STAC API's that offer the following capabilities at
 the `search` endpoint must include the relevant **conformance URI** in the `conformsTo` response at
 the root (`/`) landing page, to indicate to clients that they will respond properly to requests from clients.

ogcapi-features/README.md

@@ -5,6 +5,9 @@
   - [Endpoints](#endpoints)
   - [Examples](#examples)
   - [Example Landing Page for STAC API - Features](#example-landing-page-for-stac-api---features)
+  - [Extensions](#extensions)
+    - [Transaction](#transaction)
+    - [Items and Collections API Version Extension](#items-and-collections-api-version-extension)
 
 *based on [**OGC API - Features - Part 1: Core**](https://www.ogc.org/standards/ogcapi-features)*
 
@@ -178,3 +181,34 @@ the [overview](../overview.md#example-landing-page) document.
     ]
 }
 ```
+
+## Extensions
+
+These extensions provide additional functionality that enhances STAC API - Features. 
+All are specified as [fragments](../fragments), as they are re-used by extensions to other STAC APIs.
+STAC APIs that offer the following capabilities must include the relevant **conformance URI** in the 
+`conformsTo` response at the root (`/`) landing page, to indicate to clients that they will respond properly 
+to requests from clients.
+
+### Transaction
+
+- **Conformance URIs:**
+  - <https://api.stacspec.org/v1.0.0-beta.2/ogcapi-features/extensions/transaction>
+  - <http://www.opengis.net/spec/ogcapi-features-4/1.0/conf/simpletx>
+- **Extension [Maturity Classification](../extensions.md#extension-maturity):** Pilot
+- **Definition**: [STAC API - Transaction Fragment](extensions/transaction/)
+
+The core STAC API only supports retrieving existing Items.
+The Transaction extension supports the creation, editing, and deleting of items through the use of the 
+POST, PUT, PATCH, and DELETE methods. The full description of how this extension works can be found in the 
+[transaction fragment](extensions/transaction/). 
+
+### Items and Collections API Version Extension
+
+- **Conformance URI:** <https://api.stacspec.org/v1.0.0-beta.2/ogcapi-features/extensions/version>
+- **Extension [Maturity Classification](../extensions.md#extension-maturity):** Pilot
+- **Definition**: [STAC API - Version](extensions/version/)
+
+The core API only supports semantics for creating and accessing a single version of an Item or Collection.
+The Version Extension defines the API resources and semantics for creating and accessing versioned records.
+It is the STAC API equivalent of [OGC API - Features - Part 4: Create, Replace, Update and Delete](https://docs.ogc.org/DRAFTS/20-002.html).

ogcapi-features/extensions/version/README.md

@@ -7,8 +7,7 @@
 
 The core API doesn't support semantics to creating and accessing different versions of an Item or Collection.
 This version API extension defines the API resources and semantics for creating and accessing versioned records.
-It is done on the OGC API - Features endpoints, and ideally should evolve to be an extension there, but it is
-incubated in STAC.
+It is the STAC API equivalent of [OGC API - Features - Part 4: Create, Replace, Update and Delete](https://docs.ogc.org/DRAFTS/20-002.html).
 
 ## Methods