add links to hg.sr.ht graphql api

additionally fix typo-reference to builds.sr.ht

graphql.md

@@ -33,6 +33,7 @@ queries you perform will affect your real data!
 
 - [builds.sr.ht playground](https://builds.sr.ht/graphql)
 - [git.sr.ht playground](https://git.sr.ht/graphql)
+- [hg.sr.ht playground](https://hg.sr.ht/graphql)
 - [meta.sr.ht playground](https://meta.sr.ht/graphql)
 - [todo.sr.ht playground](https://todo.sr.ht/graphql)
 - [lists.sr.ht playground](https://lists.sr.ht/graphql)

hg.sr.ht/graphql.md

@@ -3,7 +3,7 @@ title: hg.sr.ht's GraphQL API
 toc: false
 ---
 
-**NOTICE**: This is the documentation for builds.sr.ht's new GraphQL API. For
+**NOTICE**: This is the documentation for hg.sr.ht's new GraphQL API. For
 information on the legacy REST API, see [api.md](api.md).
 
 hg.sr.ht's GraphQL API is compatible with the standard [SourceHut GraphQL API

index.html

@@ -102,6 +102,8 @@ dt {
     </dt>
     <dd>
       <a href="/hg.sr.ht">User Manual</a>
+      &mdash;
+      <a href="/hg.sr.ht/graphql.md">API Reference</a>
     </dd>
   </div>
   <div class="col-md-4">