Removed Exchange Discovery section

The /exchanges/ endpoint no longer has an entry in the oas.yaml, this PR is just cleaning up the index.html to remove this endpoint's section. 

If this endpoint should still exist then we should reject this PR and create an issue to add it to the oas file. 

#586

Author: eric-schuh

index.html

@@ -1975,17 +1975,6 @@ <h4>Create Presentation</h4>
           data-api-endpoint="post /presentations"></div>
       </section>
 
-      <section>
-        <h4>Exchange Discovery</h4>
-        <p>
-Discovery is an optional call for the [=holder coordinator=] to ensure the [=holder coordinator=] can support
-the exchange protocol requirements before calling the endpoint. Coordinators SHOULD support
-the exchange discovery endpoint.
-        </p>
-        <div class="api-detail"
-          data-api-endpoint="post /exchanges/"></div>
-      </section>
-
       <section>
         <h4>Get Presentations</h4>
         <p>