Add site and obtain a site

It's useful for whatwg/fetch#943 to be able to distinguish between a site and agent cluster key as #4734 changes how agent clusters can be keyed.

source

@@ -91744,11 +91744,11 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
   <p>A <dfn data-export="">scheme-and-registrable-domain</dfn> is a <span>tuple</span> of a <span
   data-x="concept-url-scheme">scheme</span> and a <span data-x="concept-domain">domain</span>.</p>
 
-  <p>An <dfn data-export="">agent cluster key</dfn> is an <span>origin</span> or a
+  <p>A <dfn data-export="">site</dfn> is an <span>origin</span> or a
   <span>scheme-and-registrable-domain</span>.</p>
 
-  <p>To <dfn data-x="obtain-agent-cluster-key">obtain an agent cluster key</dfn>, given an origin
-  <var>origin</var>, run these steps:</p>
+  <p>To <dfn data-export="">obtain a site</dfn>, given an origin <var>origin</var>, run these
+  steps:</p>
 
   <ol>
    <li><p>If <var>origin</var> is an <span data-x="concept-origin-opaque">opaque origin</span>, then
@@ -91762,6 +91762,12 @@ import "https://example.com/foo/../module2.mjs";</code></pre>
    domain</span>).</p></li>
   </ol>
 
+  <p>An <dfn data-export="">agent cluster key</dfn> is a <span>site</span>.</p>
+
+  <p>To <dfn data-x="obtain-agent-cluster-key">obtain an agent cluster key</dfn>, given an origin
+  <var>origin</var>, return the result of <span data-x="obtain a site">obtaining a site</span> with
+  <var>origin</var>.</p>
+
   <p>To <dfn data-x="obtain-similar-origin-window-agent">obtain a similar-origin window agent</dfn>,
   given an <span>origin</span> <var>origin</var> and <span>browsing context group</span>
   <var>group</var>, run these steps:</p>